GitLab CE SSH Ubuntu problem

I have my own server with a GitLab CE.
Ubuntu Linux Mint 19.3
GitLab CE13.12.15

I can’t use ssh for projects. Every time it asks for an unknown password after I enter the password for the ssh key. the ssh key is generated and added to the user settings. I can use this key to but not for your server.

I found a question here with the same problem. But the solution in this topic did not help me. Although I added the user to the git group and did all the manipulations in the terminal.

Any ideas?