the argument after option -f is the output path and filename of tar command - citing `man tar` :
Common options:
-C, --directory=DIR
change to directory DIR
-f, --file=ARCHIVE
use archive file or device ARCHIVE
so your correct command should look like:
`tar -czvg /home/user/BackupArchiv/snapshots/$snap -f /home/user/BackupArchiv/$buweek.tar.gz`
or maybe switch the paths since it's unclear what you want to backup and where you want it to end up - my example will generate a file:
`/home/user/BackupArchiv/backup_2020-10-08.tar.gz` for today :)