Google is now (as of a few days ago) requiring API keys on ALL map requests from any domain that has not had a map request in the past few months.
That means EVERY SINGLE NEW WEB ADDRESS where you place a Store Locator Plus map will need to have a registered Google API key. They key is free but they only allow so-many page loads per day before you have to start paying for map loads.
I saw the writing on the wall over a year ago when they “suggested” getting an API key. It was more obvious in the past 6 months when you started seeing “No API Key” warnings in all JavaScript calls if you did not enter a valid Google API Key under Store Locator Plus General /Server / Google Server API Key. Then last week they turned that warning into an error.
I’ll be updating SLP to be more obvious about this, but the short answer is that you must now have a Google API key registered for every website.