PMM does not support mariabackup at the moment, similarly mariadb server versions > 10.3 may not be compatible with xtrabackup (see Xtrabackup compatiblity with Maria 10.x (x >=3) - #2 by CTutte).
what mysql db version (minimum) does PMM support to backup with xtrabackup
MySQL 5.7 is currently supported by xtrabackup 2.4 (and yes, you should be able to use this with PMM - Supported setups for MySQL backups - Percona Monitoring and Management).
However, xtrabackup 2.4 is now EOL and you might want to upgrade your mysql server version to 8.0 which is compatible with xtrabackup 8.0