Very soon, your Google Maps may result in degraded experience if you don't act now. Google Maps Platform arrives with pay-as-you-go billing. To ensure your map in high resolution or no interruption, you must pay unless your usages are at Google "free tier" (monthly US$200 free credit).
Starting on July 16, 2018, to access Google Map APIs, you need a Google Cloud Platform billing account, besides a valid API key, This change was originally set to Jun 11. To provide more lead time, the effective date now is on July 16, 2018, next Monday!
To learn about the pricing, see https://cloud.google.com/maps-platform/pricing/
If you are new, go https://cloud.google.com/maps-platform/#get-started to get key and.set up a billing account.
If you have already had a key, see Google Guide for Existing Users.