Afnic, the registry for .FR, .RE, .PM, .WF, .PF, and .YT domain names, has a special set of rules for nameservers. If the servers do not meet their requirements, you will not be able to add the nameservers to the domain name.
If you are trying to add nameservers to a domain ending with one of these TLDs and it fails, it usually means the nameservers do not meet their rules. They have a nameserver checker here that you can use to find out if they meet the requirements. If anything fails, please speak to your hosting provider directly to have the issue corrected.