Plesk only supports LTS (Long-Term Support) releases of MariaDB. The latest TLS release is version 11.8, which is supported by Plesk. The MaraiDB 12.x branch currently still only contains rolling release with a short support cycle from MariaDB.
If you need MariaDB 12 for use with Plesk you options are either to use a docker container (and connect it as a remote database server to Plesk) or install MariaDB 12 on a remote server altogether (and connect it as a remote database server to Plesk).