This is common to European registries, the registry tries to validate the nameservers you are adding. This is most likely caused by the domain previously being active in another Cloudflare account with different nameservers. They therefore believe that the wrong nameservers are responding so prevent you from adding the new nameservers.
What to do about it?
Option 1: Wait! Cloudflare should automatically purge the zone if the nameservers are not changed to point there. This can take a while!
Option 2: Contact your registrar and request that they forcibly change the nameservers for you.
Option 3: Contact Cloudflare Support and ask them to purge the zone. You will probably have to prove ownership of the domain. To contact Cloudflare Customer Support, login & go to https://dash.cloudflare.com/?account=support and select get more help.