Your application uses an API that is linked with the PostgreSQL client C library libpq.

The version of that library must be 9.6 or older, and SCRAM authentication was introduced in v10.

Upgrade libpq on the application end and try again.

If you don't need scram-sha-256 authentication, you can revert to md5:

set password_encryption = md5 in postgresql.conf
change the authentication method to md5 in pg_hba_conf
reload PostgreSQL
change the password of the user to get an MD5 encrypted password