Relations don’t show up because they are links to entire rows and not a specific column. A lookup will work, but the source relation needs to be a single relation. If it’s a multiple relation then the lookup will be an array. You can’t place an array of multiple items into something that can only hold a single item. So the solution in this case is to change your relation to a single relation so the lookup becomes a single item instead of an array. Then you can choose that lookup column.