At startup, Xarchiver detects the available installed archivers (tar,rar,zip etc). Since Xarchiver is a frontend (a graphic user interface to the cmd-line executables) it can't create archives whose archiver is not available. For instance if you have the free edition of rar, unrar, Xarchiver can't create rar archives since unrar allows rar archives to be opened, tested and extracted but not to be created.