WHMCS version > 6.0
If you use WHMCS 8.0 make sure to install SolusVM WHMCS billing module version v4.1.2 or newer
PHP version 7.0.x or higher with ionCube PHP Loader version 10.2.x or higher
PHP version 8.1 is supported since version v4.2.1 of the SolusVM WHMCS billing module and version 8.6 of WHMCS.
Note: PHP version 8.1 is the maximum supported version currently.