The non-existent measure

I was working on my first bigger project when the serious issue appeared. I have no idea why, but I realised that I've got non-existent tact in my piece (tact number: 56, cello [violoncello]). I can't do anything with it- I can't add notes, paste anything or even delete it. There's even no pause there like it was only empty staff with bass clef. I have no idea how to fix it and it's blocking me very well from moving forward with my piece. I'm giving you the .mscz file and a screenshot of how it looks like.