Despite following this advice, the integration still isn't working, please can you have a look and see why:
Hi there @Sam
Welcome to WPMU DEV, glad to have you aboard!
It looks like you have installed WHMCS in a sub-folder of the same directory as your WordPress install. That is a type of setup that will create such issues.
As noted at the bottom of the usage instructions:
If both your WordPress and WHMCS installs are on the same server, please ensure that they do not reside in the same directory or you will have problems.
Please try adding WHMCS as an Addon Domain or a Subdomain on your main account."