This is usually caused by one of two things:
1. Maps JavaScript API not enabled
Go to Google Cloud Console → APIs & Services,
search for Maps JavaScript API, and click Enable.
2. Billing not set up
Google Maps requires a billing account. Visit console.cloud.google.com/billing
to add a payment method. Google provides a free $200/month credit — more than enough for this use.
API Key in use: AIzaSyAR57o-UG105BU2BhM5Tx5Y2gxSKvXXXXXX