Disable this validation when the client chooses the option "I will use my existing domain and update my nameservers".
In our case, when our clients choose that option we want them to be able to use any domain. Since they are not registering them with us, there is no need to validate if we have that tld, or anything.
I know there is a solution here: https://help.whmcs.com/m/85428/l/1034165-troubleshooting-invalid-domain-name-provided-messages
But it says that to be able to fix this issue, we have to add either a whois server or a price for each tld. As I have stated before, we don't really care which tld the client chooses, which means that we would have to add a whois server for every possible one (since adding a price is not an option, because we don't want the client to be able to register or transfer the domain) there are hundreds of them...