Along with the fact that the School of the Cat obviously trained female witchers, PLUS there's a female Witcher named The Dragonfly. This whole "WomAn cAn'T bE wItChErS"! Thing is just blatant sexism from the other schools because they refuse to train them or properly give them the mutations needed.

https://www.thegamer.com/theres-no-good-reason-that-women-cant-be-witchers-too/