Error: Cannot show requested dialog (SqlMgmt). A required operation could not be completed. You must be a member of the sysadmin role to perform this operation. (SqlManagerUI)
Description: Again another connectivity issue where the user name using having no permission or rights to complete the operation. Or, we can say the user is a normal or standard user.
Solution: Select and right-click Logins from Security node, from Properties window select Server Roles and select sysadmin check box.