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.
Method 1: Upgrade libpq on the application end and try again.
yum install https://download.postgresql.org/pub/repos/yum/reporpms/EL-7-x86_64/pgdg-redhat-repo-latest.noarch.rpm
yum install postgresql10-libs postgresql10 postgresql10-devel
For your OS flavour and required virsion follow the link https://www.postgresql.org/download/linux/redhat/
Method 2: 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
