EDIT - I should note this has only happened since the latest update.
I have seen some comments about this as a Steam issue, but I’m using Android and have the same or similar issue.
Each day the S.H.I.E.L.D. Ressuply window pops up, I can claim the reward (I think) and I can scroll through and see players names. I can even click players to see their teams, but the Resupply window cannot be closed.
I cannot click the button to close, and I cannot use my back button as normal either. It is simply stuck on that screen no matter what I do. I must go to my active apps, kill it and restart each day.