This is caused by WHMCS not reading the quickbooks_sync or paypal_billing_center module hooks.php file. This is usually caused after an module upgrade that encountered an error, or a migrate from a different WHMCS install.
You need to load our hooks file once in WHMCS, so it can realize our module is fully installed.
- Download the hooks.php file inside the module folder inside modules/addons.
- Upload that file into includes/hooks in your WHMCS
- Load your WHMCS admin > Setup > Addon Modules page
- Remove the file you uploaded into includes/hooks