RegisterCom Moduls not works

[Xx4everxX]

I'd edit the "Domain Registrars" For Register (or RxPortalExpress) Module with my unique key extractly

But when i go to my client's doman that registed at RxPortalExpress, WHMCS display:

Could not retrieve nameservers for the domain ***.us

Or when i register a new domain:

An Error Occurred

Could not add or update user account XXX with Registrar for the domainAdd request of ***.us

Please help me to fix this problem

thanks alot