Why am I unable to set my name servers for my .DE domain?
The central registry for .DE domains requires that all name servers respond to DNS queries before they can be set to a .DE domain. This means that not only will your name servers need to be valid, they will also need to be properly configured to serve your specific domain and have diverse IP addresses associated to them.
If you receive an error message when trying to set name servers for your .DE domain, one or more of these requirements have not been met. To check your name servers, try running a Pre-Delegation Check. This should help you pin-point the issue so you can inform your web host.
Here is more information on .DE name server requirements.