I had that too. The key (no pun intended) is to have your phone on a flat non-moving surface.

I found that I needed to orient my Canadian passport face down, covering the bottom half of the phone (iPhone 12 mini in my case). Then just let the passport sit there for about 10 seconds. You may need to play around with the position a bit, but that should work.

The Australian ETA app works in a similar way, but their instructions (and error messages) are far better.