The global setting is at Options\Settings\Media Management
If you are getting 21.2 GB on a 20/40 tape, you are indeed filling the tape, and is it looking for a scratch/overwriteable tape to continue.
If this is a standalone drive, about all that you can do is remove the mounted tape and give it a scratch to complete the backup.
You may need to split your backups and run smaller jobs on alternate days, or get a larger capacity tape drive