As of late 2021, wildcard SSL certificates must be validated via a DNS CNAME record - file based validation is no longer an option. Please note, this is a CNAME record, not a TXT record as is used for other DNS based validation mechanisms present for other services.
Unfortunately, nowhere in the WHMCS does it state that the record must be a CNAME record as opposed to a TXT record.
After placing the order you will be presented with the CNAME value and the value it needs to point to, however the information will also be present on the product details page prior to validation completion, as seen below:
Only once the correct record is present and propagated will the domain validation process be completed and certificate issued.