error/repository/instancenotsaved

This message (which doesn't seem to have a translation) occurs if there is a problem when creating a repository.

Depending on the repository type, there might be various possible causes for this error. I am not aware of any particular way to cause this error in a standard Moodle install.

At a technical level, this would occur if a repository plugin returns false from its 'create' function. If the error occurs inappropriately while developing a new repository plugin, this is the probable cause.