Hello,
As per Google documentation:
A darkened map, or ‘negative’ Street View image, watermarked with “for development purposes only” is returned when any of the following is true:
The request is missing an API key.
Billing has not been enabled on your account.
The provided billing method is invalid (for example an expired credit card).
A self-imposed daily limit has been exceeded.
You need to create billing account.
Best Regards