What you're seeing
$ stellar tx new set-options --signer sponsor --signer-weight=20
error: invalid value 'sponsor' for '--signer <SIGNER>': xdr value invalid
$ stellar tx new set-options --signer $(stellar keys address sponsor) --signer-weight=20 --build-only
AAAAAgAAAACkBZuRlmsx0id9DepMyDU50OLurzdOysfTio7sxz6HzwAAAGQAANJ2AAAAAwAAAAAAAAAAAAAAAQAAAAAAAAAFAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAAAAAIogPEBhv4JY/n4BwiL6KsNVbNk/LyVwhverKE31ymPPAAAAFAAAAAAAAAAA
What you'd like to see
This command should be able to accept an identity name, rather than just a public key.