Receiving error 'wrong key type' while JWS encoding

I am trying to JWS encode a request to the google endpoint for service accounts.

I have the JWS encode function most of the way but am not sure what to put for a key or ttl value. Can someone shed some light on this? Would love to buy you a coffee!

When I run this with any value for the key i get an error 'Wrong key type'
[Screen Shot 2022-04-19 at 7.39.53 PM.png][Screen Shot 2022-04-19 at 7.40.00 PM.png]