for jee main 2021 April session I didn't apply will they open application for April session ???

Dear student,
Yes the application window will open for each session separately, if you wish you can apply for all the sessions at a time or apply separately gor each session. The application window for april session will be opened most probably in the end if may. While the application window for other sessions, i.e. march will open in the last week of February and for may it will open in the first week if may tentatively.