Hi @Willem
According to IANA Root Zone Database .nft is NOT an officially recognised gTLD.
Unstoppables documentation Configure your Browser for Unstoppable Domains shows you need to make some configuration changes in order to access the domains they are selling, or use the Unstoppable browser.
As .nft and other Unstoppable domains are not part of the offical database, and are not resolvable via (standard) DNS, it is an invalid domain name.
This is part of the FAQ from the Unstoppable homepage:
