Have a look at the following solution : https://community.nintex.com/t5/Technical-Issues/Known-Issue-quot-K2-Service-does-not-have-Create-Ri...
I hope this helps your issue
Thank you but I'm not using SharePoint, also, I don't have any K2 Service account in my list and I don't have security tab in the category folder!
P.S. my account is am Admin and I have access to everything!