I can see message
I can see it on my site too.
I think because they change its terms.
I can see in my dashboard Google account that I didn't create billing account.
When I developed my extensions, I got API key, but today they write I haven't it.
And you can see this message when you change to "Map" map type.
And as I understand right, they give "free" $300 at all to use maps, after that there is "No auto-charge after free trial ends". It is cloud case.
As I understand, I will need to use new rules, to create billing account and link it to my project.
Before that it was 25k free maps per day (just create API key) , today 28.5k maps per month.
They write $200 each month free for display maps, it is 28.5k maps
Check terms, probably contact to its service