In order to help, we'd need you to post the score you are having problems with and precise step by step instructions to reproduce the problem. In a score created from scratch, this works fine, so presumably the specific score you are trying to edit has become corrupted somehow, and we would need to understand exactly how that happened.
BTW, it appears that for some reaosn you are entering notes into voice 4. This would almost never be necessary or advisable. If you previously entered notes into that voices then tried to delete them, this can create "holes" that won't let you enter new notes. But simply deleting the contents of the measure and starting over would fix that, as would moving the contents of that voice into voice 1.
It's unfortunate that this sort of confusion exists in the use of the word Voices, I can't think of a way to avoid this except for the user to learn the difference through posts like this.
Comments
In order to help, we'd need you to post the score you are having problems with and precise step by step instructions to reproduce the problem. In a score created from scratch, this works fine, so presumably the specific score you are trying to edit has become corrupted somehow, and we would need to understand exactly how that happened.
BTW, it appears that for some reaosn you are entering notes into voice 4. This would almost never be necessary or advisable. If you previously entered notes into that voices then tried to delete them, this can create "holes" that won't let you enter new notes. But simply deleting the contents of the measure and starting over would fix that, as would moving the contents of that voice into voice 1.
It's unfortunate that this sort of confusion exists in the use of the word Voices, I can't think of a way to avoid this except for the user to learn the difference through posts like this.
Perhaps this will help: https://musescore.org/en/handbook/voices-0
Please rightclick the measure, click "Measure properties" and check if "effective" is set to 8/8 or 4/4.