rest-api error after updating to WC 3.6.4

Please uninstall the plugin and install it again. Make sure that you have the newest version.

If that does not solve the issue, please reach out to us using this contact form: https://app.getresponse.com/feedback.html?devzone=yes with your account email and API key, and we will look into this (please do not share your data here for security reasons).