The Important note in the section "2.5.7.3. System database secret"  [1] is mentioning 3scale version 2.12 where need to be 2.13.

The note says:
"If you use MySQL 8.0 with 3scale 2.12, you must set the authentication plugin to mysql_native_password."

Where is supposed to say:
"If you use MySQL 8.0 with 3scale 2.13, you must set the authentication plugin to mysql_native_password."

[1] https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.13/html-single/installing_3scale/index#system-database-secret