When you add a new URL to track, you may see the message below: Invalid Google API Key

Or in your Unlimited or Agency profile:

The problem is coming from the Configuration of the API Key Restriction, you may have to put a HTTP referrers restriction like this one:

Please remove it! The call is made from the PageSpeed Leads server and Google checked the IP, not the domain!
You don’t need to put a restriction, only you knows the key and you can create one just for PSL.

BUT, if you really want to put a restriction in place, use IP addresses restriction with the IP 178.128.230.192. The problem with using the IP is, when PSL is growing we may need to upgrade our server to a bigger farm and the IP may changed… So you will have to change the IP again.

AFTER you remove the restriction, go back to your Profile, remove the API key, click somewhere else and copy back the API key. The message will disappear magically.

Removing the restriction will solve the error message.