You can do one of the following ways and check.
- Change Template. In most case changing template fixes the WHMCS domain checker issue. Go to Setup > General Settings.
- Delete and redo TLD Price List. Delete all TLD price and re-do the process. Go to Setup > Products/Services > Domain Pricing.
- 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.