I'm trying to create a new private key (I suppose it will be an RSA key) using seahorse, but once I insert label and strength (the other fields are disabled) the create button is still disabled.
It looks like this may be a bug in Seahorse, as I just tried to generate an RSA private key on Xubuntu 18.04, and encountered the same issue.
You can instead generate the RSA private key using the command-line:
openssl genrsa 4096
You should then be able to import this into Seahorse if you need to.