WHMCS requires a setting on the MySQL or MariaDB server for the SQL Mode. To verify this, log into cPanel then go to phpMyAdmin and click on Variables on the right top side. Search for SQL Mode and see what entry is listed there. If you see something similar to:
Then this indicates you have SQL Strict mode enabled which prevents WHMCS from completing SQL queries and will not allow new entries to be added to the database table.
To change the mode on the server, you will need to SSH with root access to your server and edit your my.cnf file, located typically in the /root folder. Add the following entry to your my.cnf file:
[mysqld] sql-mode = "NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"
If you already have a [mysqld] section, you can add the entry to that section. Then restart your mysqld service with
service mysqld restart and verify the mode has cleared by going back into phpMyAdmin > Variables and refreshing the page to ensure the SQL MODE now reads
If making the change above doesn't work, there may be an additional file at /usr/my.cnf that needs to be edited. If there is a file there, and you find the sql-mode entry in that file, you can remove the line from the file, save the file and restart the mysqld service.
If you are unsure or unable to make this change, you will need to contact support to perform the work for you, as this is a server level configuration that they will need to address for you.