A 400 error almost always means the API key was rejected or the request was malformed. Run through this checklist:

Key format — it should begin with sk-ant-. Re-paste it with no leading or trailing spaces.
Where it's saved — enter it under Settings → AI, then save before testing.
Model selected — make sure a model is chosen; an empty model field can trigger a 400.
Billing — confirm the Anthropic account has active credit, not just a created key.

If it still fails, generate a fresh key and replace the old one. That clears up the large majority of cases.