How are you trying to add a tie? what happens when you try?
Are you sure it is a tie that you want and not a slur? A tie joins two notes of the same pitch and cannot be added to notes with different pitches. A slur joins notes of different pitches.
If none of that helps, please attach your score (.mscz) file and describe in detail what you are trying to do - e.g. click on first note in measure X, press "+" key, etc.
Comments
How are you trying to add a tie? what happens when you try?
Are you sure it is a tie that you want and not a slur? A tie joins two notes of the same pitch and cannot be added to notes with different pitches. A slur joins notes of different pitches.
See https://musescore.org/en/handbook/3/ties
and https://musescore.org/en/handbook/3/slurs
If none of that helps, please attach your score (.mscz) file and describe in detail what you are trying to do - e.g. click on first note in measure X, press "+" key, etc.