deleting rests?

See: https://musescore.org/en/handbook/3/voices#delete-hide-rests
https://musescore.org/en/handbook/3/note-input#delete-notes

For most things, Delete is the key to press on your keyboard to delete something. For anything you added to your score other than a note or test, it is just gone, replaced by nothing, because it didn't need to be there to begin with. For a note, it is replaced by a rest - which is to say, you are replacing the sound by silence, leaving absolutely everything else about the score the same (in particular, not changing the timing of subsequent notes). But there is no such thing as "removing silence", pressing delete on a rest makes no real sense. Assuming your actual goal may have been to take some unspecified number of subsequent notes and shift them earlier, simply do that directly - select them, then cut and paste directly over the rest. Thus, you are quite literally replacing silence with sound.