Musescore 3 - I don't understand how to remove these extra beats

First, start with the first note after the first rest and cut and paste each continuous run of notes in the measure so that there is no rests between them. Now right-click on the measure and change the actual duration to be identical to the nominal duration.

As to why it is happening: the + indicates that the measure contains more beats than the time signature specifies. I suspect you accidentally enabled timewise note input mode. Ensure that the default step-time note input is enabled before you continue.

Those blue pluses are informational telling you that there are more beats in the measure than indicated by the time signature. There are a couple of ways to enter extra beats into a measure, the easiest way by accident is to press ctrl+shift+ a note name.

The most likely way these were introduced is that the score you are showing pictures of was imported from another source rather than manually entered by you. The fact that there are invisible rests randomly spread our in the measure leads me to believe this. The easiest way to repair this measure is probably to reenter it from scratch. Select the measure and press ctrl+del to delete it then press insert for a new empty measure.

I await your response.