Inserting and deleting rests

Voice 1 rests can't get deleted, just marked invisible, others can, just select and hit Del. You can delete the entire bar by selecting it and pressing Ctrl+Del, multiple bars by selecting the range, then Ctrl+Del.

See https://musescore.org/en/handbook/voices#delete-hide-rests and https://musescore.org/en/handbook/measure-operations#delete

As long as you keep in mind that MuseScore starts by filling measures with rests and you replace them with notes, you will never have a problem adding rests. When you delete a note it is replaced by a rest. There are some exceptions, but that is beyond what you are asking. The rests that MuseScore starts with are measure rests as opposed to whole rests.

To turn the measure rests into a long muli-measure rest press M and they will automatically be changed. Keep in mind, this will turn all measures into a single rest until something like a rehearsal mark is encountered. It is advised to not use this until you are done entering the rest of the song.

When you enter a score for a solo instrument only, it will be necessary to press M to get multi-measure rests. If you enter a score with several instruments, when you go to File->Parts to extract parts, the parts will default to multi-measure rests turned on. It is advisable to enter a complete score, then extract parts when possible rather than entering parts so you can see them in a complete score.