Voicing

You are talking about the 2nd bar, aren't you?
I can't add another not to the 2nd voice either, but I can if I delete the 1st note of the 2nd voice first, then delete the resutling rest, then add the 1st note again, now I can continue adding the 2nd note and subsequent ones (in 2nd voice)

I did the first 3 bars for you, see attached.

I find it easier to enter these 2 voices into 2 staves first and merge them later. But that is most probably due to my lack of note reading capabilities and the different clefs...

Well, what you gave wasn't a specific series of steps, but rather, the results you tried to accomplish. By specific series of steps, I mean something like "first click the first complete measure, then press N to go to note entry mode, then click the voice 2 button, then press 5, then press A, then press 4, then press D" - that way we would know exactly *how* you were trying to do what you describe wanting to do. Useful to keep in mind in the future.

But in this case, now that I see the score, I don't need to need to know the exact series of steps you tried to follow, because I can see the score is corrupt. The second complete measure appears to have too many beats, as the two quarter rests at the beginning of the measure don't belong there. So it is the corrupt score that is preventing you from getting the note onto beat 2 of the first complete measure.

Of course, that just begs the question, how did the score get corrupt? You didn't do anything wrong - nothing you do should ever corrupt a score - but rather, a bug in MuseScore is what would have that corrupted the score. However, something you did must have *triggered* the bug, so if you have any insight into what you might have done that led to the corrupt, I'm sure that would help the developers track down and fix the bug. Usually, though, when corruptions like this happen (and they do happen from time to time, unfortunately), we aren't aware of it until too late. I'm guessing it was the result of experimenting with how to use multiple voices, or perhaps it was something about how you created the pickup measure, where you might have tried something the program didn't *expect* you to try. But again, obviously, nothing you did should have resulted in a corrupt score, so it's definitely a bug.

Anyhow, the solution when corruptions like this happen is to delete the offending measure(s) entirely, insert new ones, and re-enter their contents. In this case, it's that first full measure that is corrupt. Sometimes, you can get away with copying and pasting the old contents into the new measures, if you avoid copying the corrupt part. In this case, voice 1 seem fine; the corrupt seems limited to voice 2. And unfortunately, there is no direct way to just copy and paste one voice. So you'd have to re-enter both voices in that measure after deleting the measure and inserting a new one in its place. If it's any consolation, had you been entering this on two staves as in the original and the corruption was just in one staff, you could have copied and pasted the contents of the good (top) staff and only had to re-enter the bad (bottom) staff.