I believe the account for an API key is different than the account for using the web-app at claude.ai. If it’s a brand new API key, they often require a small pre-payment. Model providers do this to make sure API key holders are legitimate. There are malicious actors who might spin up thousands of new keys and spend on credit.
In any event, I recommend going into the Anthropic billing portal and pre-paying $10 by clicking “add funds”.
I suspect this will fix the issue. If not, I have a few other ideas.