I've had this issue as well. Best case in the future is to take a screenshot of your boarding pass using the united app. Also get a mileage plus account and link it to the united app to save boarding passes.

What you want to do now is go to a gate agent with your confirmation number. They can pull up your boarding pass from a previous flight and print it out, then take a picture and submit it.

I tried everything I could think of to retrieve a boarding pass with no success until talking to a gate agent. I then signed up for the mileage plus account and make it a habit to take a screenshot as soon as I check in.

Now I submit the screenshot, a picture of the pairing, the flight details and seat number and a brief description that basically says I was in a middle seat so I get 50% add pay per the UPA.