Why is my max loss blank, and what is "undefined risk"?

Because the position genuinely has no worst case. A naked short call can lose as far as the underlying can run, so there is no number to put there.

The app refuses to invent one. Its own note reads: "Undefined risk. There is no worst case for this position — the loss grows as far as the underlying moves against it, so there is no number to put in that box and no denominator for an R-multiple. Buy a wing or hold the underlying and it becomes a figure you can size against."

A blank here is a finding, not a gap. Putting a large stand-in number in that box would give every R-multiple on that trade a fake denominator, and the R would look reasonable while meaning nothing. The same applies upward: a long call's max profit is blank because it is unbounded.

You may also see an amber note that a position has the same downside as a naked short put. A covered call is the usual one. "Covered" describes your margin, not your risk — the stock can go to zero underneath it — and the app says so rather than letting the word do the reassuring.