After sending the following GET request (with API key added in place of [API_KEY] to the Google Pollen API, I get the a response that information is unavailable for this location:

Request

https://pollen.googleapis.com/v1/forecast:lookup?location.longitude=51.5085&location.latitude=-0.1257&days=1&plantsDescription=true&key=[API_KEY]

Response

{
    "error": {
        "code": 400,
        "message": "Information is unavailable for this location. Please try a different location.",
        "status": "INVALID_ARGUMENT"
    }
}

Docs
According the following docs, my location in United Kingdom is supported by this API.
https://developers.google.com/maps/documentation/pollen/coverage

I have tried removing the optional keys plantsDescription. I have tried many other variants of lat long coordinates in supported countries. I have tried adding the coordinates as a comma seperated value under a single ‘location’ key.