I have been studying slavery in Islam, I have a question on rights of woman master with her slave, if a man can have sex with his slave why a woman cannot have with her slave?
Because Allah has described which sexual relations are permissible and which are not.
والذين هم لفروجهم حافظون إلا على أزواجهم أو ما ملكت أيمانهم فإنهم غير ملومين فمن ابتغى وراء ذلك فأولئك هم العادون
And they who guard their private parts. Except from their wives or those their right hands possess (concubines), for indeed, they will not be blamed. But whoever seeks beyond that, then those are the transgressors.
This verse is (grammatically) addressed to men and allows for sexual intimacy between:
A man with his female wife. And hence a woman with her husband.
A man with his female slave. And hence a female slave with her master.
Everything else is forbidden as it falls under "whoever seeks beyond that". Hence sexual relationship between a slave man and his female owner will be forbidden.
Arguing that the permission also applies to a female owner with a male slave requires analogy, and such an analogy is invalidated by Ijma'. Analogies like this can not supersede Ijma' and especially the Sahaba knew better how to interpret and apply the Quran.
أجمع علماء المسلمين من الصحابة والتابعين، ولم يختلف في ذلك من بعدهم من الفقهاء: أن المرأة لا يحل لها أن يطأها من تملكه
The Ulema of the Muslims from the Sahaba and the Tabieen have a consensus on this, and none after them from the jurists have differed on this: That it is not permissible for a woman to have intercourse with her slave
— al-Istidhkaar - Ibn ‘Abd al-Barr