Which version of ERPNext are you using. In the latest one Workflow is there, so htat employee can make application as open status and later one who have Leave Approver Role can Approve & Rejected.
Test Employee has created a Leave Application. When I log in to the account that is the Leave Approver for Test Employee, I get a “Submit” button, and no option to “Approve” or “Reject”. When I try to “Submit”, I get the same error as the OP above.