@Sean

Thanks a lot Sean !
It appears indeed that the credentials generated was not the right type (I may have not selected the right option in google api console). Worked better the second time !

Marvelous results of the module

Dear Sean,

It seems the node auth_and_test.js command throws an error on my side :

pi@raspberrypi:~/MagicMirror/modules/MMM-GooglePhotos $ node auth_and_test.js
/home/pi/MagicMirror/modules/MMM-GooglePhotos/auth.js:32
  const oauthClient = new OAuth2Client(key.client_id, key.client_secret, key.redirect_uris[0]);
                                           ^

TypeError: Cannot read property 'client_id' of undefined
    at new Auth (/home/pi/MagicMirror/modules/MMM-GooglePhotos/auth.js:32:44)
    at Object.<anonymous> (/home/pi/MagicMirror/modules/MMM-GooglePhotos/auth_and_test.js:16:14)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:623:3)

It seems it cannot read my client_id from credentials.json file… do you have an idea ?
Thanks a lot for your help !