you ideally import this in another smartcard device/second Nitrokey if you do not have the original. Basically you have to use the “bkuptocard” command, which can be reached through the ‘–key-edit’ dialog.
That is to say you use the key stub in your keyring.
gpg --card-edit <keyid>
You maybe have to put in the whole path to the file.
I hope this helps. This backup option is not the best thing in GnuPG in my opinion