I think I found this mentioned on another post, but the ETA app doesn't always work well on android phones. In my case, the issue seemed to be that my phone's home button is the same spot as the app's camera button.
I ended up taking the needed photos ahead and uploading them as needed, on my phone but not through the app. I received my approval 4 minutes after hitting send.
Note that you must pay in your own country's currency. Today's cost was $22.82.