Thank you very much for your response.
InnoDB: Unsupported redo log format. The redo log was created with MariaDB 10.3.27.
# xtrabackup --backup --throttle=100 --target-dir=/media/server-backup.s3ql/innodb
xtrabackup: recognized server arguments: --datadir=/var/lib/mysql --tmpdir=/tmp
xtrabackup: recognized client arguments: --socket=/var/run/mysqld/mysqld.sock --backup=1 --throttle=100 --target-dir=/media/server-backup.s3ql/innodb
210531 14:53:59 version_check Connecting to MySQL server with DSN 'dbi:mysql:;mysql_read_default_group=xtrabackup;mysql_socket=/var/run/mysqld/mysqld.sock' (using password: NO).
210531 14:53:59 version_check Connected to MySQL server
210531 14:53:59 version_check Executing a version check against the server...
210531 14:53:59 version_check Done.
210531 14:53:59 Connecting to MySQL server host: localhost, user: not set, password: not set, port: not set, socket: /var/run/mysqld/mysqld.sock
Using server version 10.3.27-MariaDB-0+deb10u1
xtrabackup version 2.4.22 based on MySQL server 5.7.32 Linux (x86_64) (revision id: c99a781)
xtrabackup: uses posix_fadvise().
xtrabackup: cd to /var/lib/mysql
xtrabackup: open files limit requested 0, set to 1024
xtrabackup: using the following InnoDB configuration:
xtrabackup: innodb_data_home_dir = .
xtrabackup: innodb_data_file_path = ibdata1:12M:autoextend
xtrabackup: innodb_log_group_home_dir = ./
xtrabackup: innodb_log_files_in_group = 2
xtrabackup: innodb_log_file_size = 50331648
InnoDB: Number of pools: 1
InnoDB: Unsupported redo log format. The redo log was created with MariaDB 10.3.27. Please follow the instructions at http://dev.mysql.com/doc/refman/5.7/en/upgrading-downgrading.html
Found that log format diverged from MySQL: InnoDB: Unsupported redo log format. - #2 by Michael_Coburn