A reference load is required for calculation of the Quarter wave coupler characteristics (defaults to 50 Ohms).
If you are looking for the 3.02db holy grail coupler, then try Er = 9.6, height = 2mm, spacing 0.0009mm, width = 0.628mm
and ABOVE ALL thickness = 0000mm. You will not get a microstrip 3db coupler using this separation with any practical line thickness. Well a graphene film is a possiblity?
and don't forget to align the graphene planes correctly.
However if you are looking for an edge coupler on a real substrate, then choose a substrate from the catalog.