Need help with Gmail set up

Hi, I set up through Google develop console (my account) but it is for a client. It says I have everything correct but that I need to “grant permission from Google”

Because this is through MY dev account and clients hire us to do these techy things and don’t have a dev account, it does not seem to work. We want the client’s email as sender, definitely NOT mine.

What am I possibly missing? The grant permission does not work regardless of whether I try to do it while logged into my Google account or his.

The page I need help with: [log in to see the link]