If you schedule a training event using our Employee Training Management for SharePoint, you can set an enrollment deadline. After the enrollment deadline users are no longer allowed to enroll in the training.
We do not check if the enrollment deadline is before the end date of the training, but if you have this requirement, you can add a validation rule in the “Scheduled Trainings” list. Go to the List Settings, Validation Settings and add a rule.
For example: =[Enrollment Deadline]<=[End Time]
If you don’t need to set an enrollment deadline for each training, you can also make the field optional in the list content type and leave it empty to allow users to enroll at anytime.