@epnyc Are you sure the "Client credentials JWT populate" is the type of lambda you want to create and not a "JWT populate" lambda?
If you are looking to assign a "JWT Populate" lambda, you need to go to Applications -> Your Application and select to the JWT tab and then select the lambda under the Lambda settings. (You will need to create a lambda of this type in Customizations -> Lambdas)
If you are looking to assign a "Client credentials JWT populate" lambda (sounds like this is what you created), you need to **Tenants -> Your Tenant ** and then select the OAuth tab and choose the Client Credentials populate lambda.