What is the error code -5 when saving in Geometry Dash?
It is a known issue, as stated by several Steam Community posts (links given through the article).
According to a developer of the game in this Steam post, you need to login again to your account (refresh login), or try to change your password from the help page if that doesn't work. The user of this other post tried refresh login and it worked.
A player from this post says it happens after he changed his username: when you do that, you have to log off from your account, on all your devices, and then log in again with the new username.