API keys mismatching between your Magento 2 installation and the web service user in Adyen Customer Area.
- Regenerate the api key and flush cache in Magento.
- If you are in LIVE make sure that the live endpoint prefix is correct.
- Log into the Adyen customer area and check in the web service user that the Client-Side Encryption library is green. If it is not green then change it to a different version and see if this resolves the issue.