If WHMCS Freenom module says "Error: Not enough money." it is probable that it it trying to charge your freenom account. Are you sure that you want to be charged for the .tk or similar domain? Charge can be 7 and more.

One can fund Freenom account and this errror should disappear and domain be registered for the price of around 7+

To prevent registering .tk as a PAID domain, and register it for free one have to change domain's price in WHMCS to be 0.00. The freenom module is verifying that the price is 0.00, else it submits paid registration apparently.