This could be due to a few things but the most common issues are:
- The proper APIs aren’t enabled for your Google Key, refer to Google Maps in the documentation.
- Your sites URL isn’t allowed access to that specific API key you’ll need to generate another one and not lock it to your domain or make sure https://your-domain-here.com/ is assigned to said key in your Google Platform account. Check out https://productforums.google.com/forum/#!topic/maps/Lcx4uBRl7uA
- You’re seeing a “Development Only” layer, you just need to enable Billing on the Google Cloud Project here, learn more at https://developers.google.com/maps/gmp-get-started. Once you’ve done that the “Development Only” layer will be removed.
- Lastly, it could be the actual listing address data entered that could be bad or incorrect, which is causing the maps to choke and fail.