Sorry to hear about your difficulties. I'm not able to recreate this on my iPhone. Does it happen in all projects? Would it be possible to share a screen shot of the issue?
The most common case when the Remove background button is greyed out is when the image hasn't finished uploading. During that time, there is usually also a yellow uploading icon on the top-left of the image.
Agree that this is not self-explanatory, and we are considering improvements to the user experience.