I think you definitely identified a bug. Possibly with the new ability to rename columns in combination with using a rollup against a renamed relation. Did you happen to rename the relation column after you created it?
I was able to reproduce the problem. First I created an identical relation and then created a rollup column against that relation. Everything worked fine and I was able to get a count of the true reserved checkboxes. It wasn’t until I renamed the relation column that I could no longer create a rollup against that relation with the ‘Done’ button being grayed out.
Deleting and recreating the relation with the name already set would fix this issue.
@Mark I think we definitely have a bug here. I have a copy of the app in this state if you need it.