The Encode JSON Web Token Assertion creates a compact, URL-safe message as a JWT token that is represented using the JWS or JWE Compact Serialization. The Encode JSON Web Token Assertion uses only a private key to sign.
The Create JSON Web Key Assertion creates a public key and so cannot be used for signing. You can use this site to generate a key-value pair https://mkjwk.org/
Then use a context variable as the private key to encode.
You can then use the Decode Json web token assertion to decode the payload
Here is an example of an assertion that you can use. Russ.xml