In the weeds with Pardot API Logins failing
Logging into the Pardot API failing even though the username, password, and API Key are correct?
Using either of the Upper tiers of Pardot? The API isn't available to the base tier.
The user may have been created automatically from a Salesforce user which are not allowed to use the Pardot API. See how to determine this as well as suggested best practices for API users.
API Users should be created in Pardot, not synced to SFDC, and be unique for each API connection.