Error 401 authorisation required

"The 401 Unauthorized error is an HTTP status code that means the page you were trying to access cannot be loaded until you first log in with a valid user ID and password. If you've just logged in and received the 401 Unauthorized error, it means that the credentials you entered were invalid for some reason."

So you are trying to save your score online, on musescore.com?

a) You have to be logged in on musescore.com, else how can MuseScore know which user account to save your score to?
b) If an online Save simply doesn't work, then save your score locally to your PC or laptop.

I just had this error as well. I fixed it by navigating to the 'Home' menu in the top-left corner of the MuseScore application (not the website!), clicking on 'My Accounts', and clicking the grey 'Sign Out' button under where it has your MuseScore account, then signing back in. If it turns out that you just weren't signed in, then all you have to do is sign in. I hope this solves your problem! I tried to be as clear as I could, but I'm not great at explaining things, so let me know if you're confused!

Simply close and reopen the program. I have that problem even with my session open, but apparently the session is interrupted when you put your computer to sleep.

Before doing so, press cmd+a to select your entire score and cmd-c to copy it. When you open the program again you can overwrite your previous work (or do it over a new file) with cmd+a and cmd+v to paste your last work. This way you will not lose the last changes made.

This is the way it worked for me ;)

mine did that today as well; closing and reopening the app fixed it for now. I tried the "copy all and paste" solution that Alejandro suggested, but for some reason it didn't work lol. at least I only lost a few lyrics since the last successful save.

I am trying to Publish my score and am having similar problems.
i.e. Publish to MUsescore.com tab from within Musescore 4.0, (not Save On-line)
Get error 401 Authorization required.
I tried again (though shouldn't have had to) having logged in to My Scores, but no joy.
I tried with a different score. No.
Please could someone advise?

Yeah, I get this a lot. I am logged in through the desktop app to musescore.org, even have the cloud symbol on the score tab and when I save to the cloud I get the error message even though it seems to be saving. Ugh. I liked it when it would say something like "we are saving it and will let you know when we're done". I would even get emails! good times.

sighning out and back in didnt work for me pls help