Google contacted me because payment service provider, Shopify, has been declining transactions for Buy on Google. As a result, they have paused the Buy on Google program on Merchant Center account.
How can I resolve the error for “Invalid API key” ?
Consider adding PayPal to your Google Merchant Center account to accept payments. I was not aware Shopify accepts payments directly from Google Shopping via the Buy On Google Program, but if that is the case, then you need to contact Shopify support as that is a technical issue. Not something the community can fix.
Can you confirm that in Google Merchant Center > Gear Icon > account linking > platforms, Shopify is linked as a payment platform?
If you are using Shopify as the payment gateway, then I recommend contacting Shopify support.