Problem
When upgrading to K2 5.6, there is an error shown in the Configuration Analysis for the K2 Core component:
Could not create login: System.Data.SqlClient.SqlException (0x80131904): User does not have permission to perform this action.
Solution
Run the setup manager again with the 'Configure' option using an account that has DBCREATOR and SECURITYADMIN server roles on the SQL server where K2 database will be installed.
Please see the Required Permissions here: https://help.nintex.com/en-US/k2five/icg/current/Content/Plan/Required-Permissions.htm?Highlight=dbcreator