This error occurs when the approval group is not assigned to the user. To assign an approval group you can do the following-
Go to the User Profile menu> Select T&A Rules tab> Click on OD Approval option select the the group> Click on save button.
If the Approval group is not created you can create it as follows-
1.) Click on HRView Settings option.
2.) Select Privilege Under Master configuration tab then Click on Add New button.
3.) A pop will appear. Now, Type the of the group in Name and Tick the Status checkbox and from Privilege type select group privilege then click on save button.