When I try to restore a backup from sqlserver 2008 R2 to sqlserver 2008, I have the following error:
There's a way to make a compatible backup ?
You can't because the structure is incompatible (can't go backwards).
Look at solutions like SSIS transfer between two connected servers, or if the database is small, script out the create db objects + insert data statements and execute on the other.
Or purchase some tool e.g. from Redgate for this task.