Downgrade MySQL 8.0.20 to 8.0.19 on Ubuntu for using XtraBackup
Xtrabackup 8.0.12 should work with MySQL 8.0.20
Percona xtrabackup 8.0.12 release

Percona XtraBackup 8.0.12 now supports backup and restore processing for all versions of MySQL up to and including 8.0.20; previous versions of Percona XtraBackup will not work with MySQL 8.0.20 and higher.