Google Maps shows “For development purposes only”
Google Maps is no longer free. You have to associate a credit card so that you can get billed if your site has requests that exceed the $200 credit they give you monthly for free. That is why you get the watermarked maps.
You got the error as you did not set a billing account with credit card information to comply with the new pricing structure for the Maps API. All new API projects are required to setup a billing account with credit card information.
For more information, click here