Error: Invalid username. Lost your password?

Hi, i have setup wpmu multisite provisioning at both WHMCS and Wordpress Multi, and got them network activated. When accept order in WHMCS it suppose to create new site automatically but im getting error. Please refer to the attachment image.

I have enter username into "Product Administrator" under product/services->Module Settings. Im confuse should i use Wordpress multisite superadmin user or WHMCS admin user for this field, but however......both WHMCS and Wordpress im using the same username, so the error should be cause by i use the wrong username.

Please help. Thanks