The issue is because you haven't created the billing account for your project.
Without billing account by default good maps API will allow only 1 map load per key per day.
You can follow these steps to resolve the issue :
- Go to Google Cloud Platform Console.
- Enable billing account for your project.
- Select the project that contains the API you want to review.
- From the list of APIs on the Dashboard, click the name of the API.
- Near the top of the page, click Quotas or Usage and set the limit according to your usage.