The best I can say is follow the directions step by step very carefully here:
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.
Click Update. Now your game should be authorised to be accessed from preview mode as well as when you publish it.