How to address the failure: "Each username’s first 8 characters must be unique"

Symptoms

When you create or transfer an account, you may receive the following failure:

Account “$USER”: Account Restore Failed: 
“Account failure: Failed to create the account: (XID 6abkte) 
The name of another account on this server has the same initial 8 characters 
as the given username ($USER). 
Each username’s first 8 characters must be unique. at 
/usr/local/cpanel/Whostmgr/Accounts/Create.pm line 623.”

Description 

If you are using MySQL or PostgreSQL, the first 8 characters must be unique on the system. However, when using MariaDB, this is not a limitation.

Workaround

1. Upgrade to MariaDB as your database server:
   How to upgrade MySQL/MariaDB
   Please note, if MySQL 8 is installed, you will not be able to switchover to MariaDB. You can view further details on this here:
   Can I downgrade/upgrade from MySQL 8.0 to MariaDB?
2. Disable the "Require a username prefix on names of new databases and database users" Tweak Setting at WHM "Home / Server Configuration / Tweak Settings / SQL".