I can't change classroom/time table to a course, the system is blocking me, how come?

If in an attempt to change the time/classroom to a regular course, operating from the class schedule of the course, the system blocks the continuation of the procedure by showing this error pop up,

means that instead of directly moving the already existing lesson to the old time/classroom i.e., the red box, a new one is being created at the new time/classroom, which will be added to the existing one, thus causing an inconsistency with what was specified in the course creation form.

Then, instead of creating a new lesson, you will have to directly move the already present lesson (i.e., the red box) to the new time/classroom and thereafter check the schedule.