l have heard that during ablution it is necessary for ladies to cover their head otherwise their ablution will not be performed. but i have searched and didn't get any exact hadees related to it.
As we take a look on the bases of islam. Islam not allowing a woman to discover her head in so places there is an irshad "the woman who never covers their heads are the wife of iblees(shitan)." The thing you said ablution is not performed while women head is uncovered...? So how can we perform ablution with uncovered head? Although your ablution is performed but it's good to cover your head not only for ablution for all the time. Thanks
In https://islamweb.net/en/fatwa/89914/ it says:
It is not an obligation on a woman to cover her head while performing ablution except if she is in the presence of marriageable men.
If the lady can be seen by non-mehrams while performing ablution then she needs to cover her head. Otherwise it is not necessary to cover her head since ablution itself is not part of prayer, ablution is valid as long as its obligations are performed which are listed in the Quran.
O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles.