Thank you for submitting the issue you encountered. We are unaware of any issues with Nintex Workflow Cloud integrating with specific Salesforce editions. However, from basic research it appears that there is a "Salesforce Professional" edition with no API access so that may be the problem. Are you able to investigate whether your edition is with or without API access?
https://help.salesforce.com/HTViewSolution?id=000005140
https://developer.salesforce.com/docs/atlas.en-us.202.0.salesforce_app_limits_cheatsheet.meta/salesforce_app_limits_cheatsheet/salesforce_app_limits_platform_api.htm