"My friend studies Psychology."
Translation:Η φίλη μου μελετά Ψυχολογία.
«Ο φίλος μου μελετά Ψυχολογία» is one of the accepted alternatives.
I’m not sure why the system might have rejected it in your case.
If it happens again, could you perhaps make a screenshot, upload it somewhere, and post a link to it? That may help us determine what might have happened.