The playground answering proves the key is valid not that the account can be billed. Two different things at most providers. The playground runs on whatever trial credit the new account came with and the API behind your plugin needs a payment method or prepaid credit on the account, otherwise every call comes back as a quota error.
And most of these writer plugins swallow that error. The box just stays empty.
I got this on a dog training LMS i'm building, key fine in the playground. Nothing in the plugin and the usage page said insufficient quota with zero spend.
So open the provider's billing or usage page, add a card or a few dollars of credit and press Generate again. Your config can stay.