You shouldn't need a looked up value setting but it should work if you have a valid relationship between two tables. That requires that the right match fields be linked in the relationship and the right matching values be entered into those fields. A looked up value setting will not change what is shown on records created before you added this field option but would work for any new receipt records you create provided the relationship details are correct.
In your case, the COS IDFK is blank in the Payment table so there then is no valid link from payments to test lookup.
Once you have a valid relationship working, you can just add the TOH field from test lookup to the portal row and it should display the current TOH value without any need for a TOHID field in payments.