There might be different cases
-> When leave request created first check workflow is triggered and it's pending with that manager for approval or not in t-code SWIA.
-> Then check with the manager logon id in t-code SBWP i.e SAP Inbox.
All shows fine, then from portal side check with portal person UWL configurations are done or not.
Which workflow your using ? If employee doesn't see the approver during leave request creation it will not allow to raise the request it self.