Keys

Can you attach the score you are having problems with?

My guess is you are working on a score for transposing instruments - eg, trumpets, clarinets, or saxophones. In which case, because MuseScore needs to automatically transpose the key for each instrument, you need to add the concert key signature you want. So if you want Eb for trumpet - which is Bb for alto sax, Ab for horn, etc - you need to add the correct concert key, which is Db.

If that doesn't answer your question, then indeed, please attach your score and explain what you are trying to do in more detail, so we can assist better.