Removing a note or a rest

For exemple if you are in 4/4 you have 4 times, you can have notes or rests but in total you must have 4 times. But if you "remove " a rest , for ex , a quarter rest , you get only 3 times, and it is impossible, so MuseScore don't accept to remove a rest.

You can remove a note, for ex a quarter note, and MS put, at the place of the note ,a quarter rest because the total must be 4 times.
If you have for ex , quarter note-quarter rest-quarter note-quarter rest, and you want to make "diseapear" the first quarter rest ,you can put a new note at his place, or transform the first quarter note in an half note (typing 6)

You delete rests by overwriting with notes.
For instance, you have a rest on the third beat. You don't want the rest there.
So... what's the alternative? Why a note, of course!!

Now, if you wish to shift notes following a rest to overwrite it, use copy/paste to time shift a passage.
See:
https://musescore.org/en/handbook/copy-and-paste#notes

Read the handbook and search the forum for many discussions concerning deleting rests, inserting notes.

Regards.