On the tile titled "Start your Stadio Pro subscription" I hit next, get the dark overlay with a spinner. The white box pops up at the bottom with a green spinner at the top, this never loads. I cannot complete the subscription, and therefore cannot use the service.
I have tried clearing cache, data, uninstalling. I've also tried clearing cache and data for Google Pay, Play Services. Nothing I've tried has gotten me past this point.
@GingerRage Someone had this issue before, and what she did was to activate code/subscription through another phone. Once it's activated,she was able to open the app just fine from her phone. I know it's not ideal, but in case you have access to another phone, you can try that method. Just make sure to log off from that other phone once you're done.
Other than that, you should contact Stadia support like what @unka12 said.
It ended up being cached data on Google Play Store. After adding a new payment method and forcing the retrieval of new data, this resolved it. I hope this helps someone else!