Hrm.
That would seem to point to the API key? I wonder if perhaps you copied and pasted it, and a space was included as the first or last character? That would cause it not to match what Google expects.
What I generally do to check whether that has happened is pasting into Firefox’s search bar, highlighting everything there using Ctrl A, then if there isn’t a space at either end, using Ctrl C to copy it and paste it elsewhere (in this case, Discourse).
There’s nothing particularly special about Firefox’s search bar, though; the important bit is having things constrained to a single line where you can easily see both ends.