I launched Next.js anew (the latest version as of June 20th, using app router) and installed the beta version of Auth.js. The official website recommended the beta version unless there was a special reason, because breaking changes were made when going from version 4 to the beta version.
From there, I followed the instructions on the official websites for Next.js and Auth.js and created a page that simply placed a Google Sign-in button in the center of the top page.
When you press the “Sign in with Google” button in the attached image, the error message at the top is displayed.
I have attached screenshots showing the settings and code on the Google Cloud console.