Concentration in Salah is very important as far as I know.
But is it a sin not to concentrate in Salah?
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Short Answer: No.
The presence of the heart or concentration in Salah is one of the most important parts of Salah and any other worships, so that worship without it will be spiritless.
Imam Khomeini says:
The presence of the heart is the key to the closure of perfections and a main door leading into actual felicity and in hadiths from the Prophet and ahl-u-albait (A.S.) it is mentioned very much and to less politeness, such importance is given. 
The presence of the heart is one of the conditions of acceptance and perfection of Salah, not its correctness. That is, if anyone concentrate during his/her Salah, he/she has performed more complete worship.
The Prophet (pbuh) says: "Sometimes one second or one third or one quarter of Some Salah's, will be accepted, and some Salah's will be wrapped up like an old gem and will be attacked on its owner, and you will not have your prayer except Salah with presence of your heart. 
 Salah, Imam Khomeini, p. 31, Institute for the regulation and publication of works by Imam Khomeini
 بحارالانوار، علامه مجلسی;، ج 81، ص 260، مؤسسه الوفأ
CONCENTRATION is defined as the mental effort you direct toward whatever you're working on or learning at the moment. in another definition it is the ability to direct one's attention in accordance with one's will.
when we try to understand concentration in prayer, what we find is that the arabic word khushoo is used to describe it in quran as well as the sunnah.
KHUSHOO is generally defined as calmness, serenity, tranquility, dignity and humility. in a more contextual definition it is when a person's heart and thoughts are humble and focused on Allah the exalted and glorified.
Ibn al-Qayyim (may Allaah have mercy on him) who said : The khushoo’ of true faith is when the heart feels aware and humble before the greatness and glory of Allaah, and is filled with awe, fear and shyness, so that the heart is utterly humbled before Allaah and broken, as it were, with fear, shyness, love and the recognition of the blessings of Allaah and its own sins. So no doubt the khushoo’ of the heart is followed by the khushoo’ of the body. (Al-Rooh, p. 314)
thus, now the question is, is it a sin not have khushoo in prayer? (i intent to use the word khushoo as it is more relevant, though it is the same as concentration)
EVIDENCE FROM QURAN (to concentrate in prayer)
in order to understand if it is a sin not to have khushoo in prayer, let us first look at verses in the quran that tells us to have khushoo in prayer ;
They who are during their prayer humbly submissive [23:2] (sahih international)
Those who offer their Salat (prayers) with all solemnity and full submissiveness [23:2] (mohsin khan)
(note : i have used two translations just to give it more clarity and so that we understand better, insha Allah)
the importance of having khushoo in prayer is further empasised more in the following verses ;
without concentration (khushoo) we can't be devoutly obedient. from the above verses it is very clear that Allah the mighty and sublime has instructed us to have khushoo in prayer.
EVIDENCE FROM SUNNAH (to concentrate in prayer)
to know if it is a sin not to concentrate in prayer, let us look at authentic narrations from our beloved prophet Muhammad (P.B.U.H) instructing us to have khushoo (concentration) in prayer.
Narrated Abdullah ibn Sunabihi : Messenger of Allah (ﷺ) say : Allah, the Exalted, has made five prayers obligatory. If anyone performs ablution for them well, offers them at their (right) time, and observes perfectly their bowing and submissiveness in them, it is the guarantee of Allah that He will pardon him...(abu dawud)
if we do not have khushoo (concentration) in prayer, we are prone to commit mistakes and errors. without the proper khushoo we will not be able to perfect or complete the prayer and this is further strengthened in the following hadith ;
the prophet (P.B.U.H) said : "the worst thief is the one who steals from his prayer" . the sahaba asked : "how can someone steal from his prayer?". He answered : "by not completing his ruku and sajdah with perfection". (sahih ibn khuzaymah)
there are various authentic reports confirming the importance of khushoo in prayer. it is a well established fact in sunnah that prayer has to be done with utmost concentration. there are hadith about avoiding what might distract you in prayer, this is also done just to maintain perfect and complete khushoo. [refer : sahih muslim 556c]
from the above mentioned evidences we can have an understanding about how important it is to have concentration (khushoo) in prayer. it is also understood that when Allah the mighty and sublime and His messenger Muhammad (P.B.U.H) has instructed us about something, we have to follow as it is. and not doing so, not following the commandment of Allah and the instruction of prophet (P.B.U.H) is considered a sin.
[ the answer is of on my understanding that what you intended by not to concentrate in prayer, meaning it is done deliberately and intentionally. we all know how difficult it is to completely submit and concentrate but that does not mean that we have to stop trying. ]
FOR MORE CLARITY
One of shaitan’s most significant plots is to divert the people from the prayer by all possible means. those who do pray are subjected to his whispers so as to deprive them of the joy of this worship and to cause them to lose the reward for it. when there is no concentration or khushoo in prayer then it will be more easy for the shaitan to mislead us.
Indeed khushu will be the first thing to disappear from the earth in these last times that we are living in.
The first thing of your religion that you will lose is the khushu and the last thing that you will lose of your religion is the prayer. There may be a person praying who has no goodness in him, and soon you will enter the masjid and not find anyone who has khushu. [Recorded in al-Madarij 1/521]
may Allah the mighty and sublime grant us mercy and guide us to the straight path.
Allah knows best