WHMCS Domain Checker not Working

You can do one of the following ways and check.

1. Change Template. In most case changing template fixes the WHMCS domain checker issue. Go to Setup > General Settings.
2. Delete and redo TLD Price List. Delete all TLD price and re-do the process. Go to Setup > Products/Services > Domain Pricing.
3. Reconfigure Domain Register Module. Configure again your Domain Register Module. Go to Setup > Products/Services > Domain Registers.

Hope one of these methods will solve the issue.