Merge access levels are restricted to a single instance per Protected branch

I wanted to create protected branches. But I got the following error message:

What does it mean and how can I create protected branches?
My gitlab version is 15.3.2.
Thank you for any help.

I’d say it means you can select only single item in Allowed to merge

No matter which item I choose in Allowed to merge, the result is the same. I’ve tried all possible combinations, but got the same error message.