The best I can say is follow the directions step by step very carefully here:
https://www.scirra.com/manual/175/google-play
Especially the part about enabling access in preview mode, copy & pasted below.
[quote:y2i36cq9]Enabling access in preview mode
When testing your project, your game will typically run from localhost or a local IP address for preview-over-wifi. Since this is is different to the published URL of your game, Google Play Game Services will block access unless you add the preview URLs as permitted locations.
To do this, in the Developer Console under Game details, find the header that says API CONSOLE PROJECT and follow the link to the API console project for your game. In the API console, select APIs & auth, then Credentials. Notice the Javascript Origins field contains only your final URL; this also needs to contain any preview URLs to be allowed access. Click Edit settings. Under Authorized Javascript Origins, make sure the final URL and any preview URLs appear on separate lines. Note normal preview by default happens on http://localhost:50000, but you also need to add any preview-over-wifi addresses you use, such as http://192.168.1.3:50000. For example, the completed Authorized Javascript origins could list:
http://www.mywebsite.com/games/myawesomegame
http://localhost:50000
http://192.168.1.3:50000
Click Update. Now your game should be authorised to be accessed from preview mode as well as when you publish it.