We just had to submit it for my mom, and the picture taking can be extremely difficult. You basically need a flat white wall with no patterns or lines, controlled lighting that is I would guess around 5000k color temp. Phone automatic mode with 2700k (incandescent) lighting will not look white enough. We had to edit the picture to remove a line and brighten up the white wall, luckily it took it or we'd have had to go find a flat wall that was white, probably get like a studio drop cloth and set up my lightstands and off camera flash (on camera left too harsh a shadow to the side when held portrait style) and probably would have used RAW and set the white balance to the "white" drop. Anyway, if you don't happen to have a completely white wall with no patterns or lines, and don't know either photography or photo editing - it can be "impossible" to get a picture they'll take.
Personally I'm probably going back to the post office and seeing if they can do it like when I first got my passport in 2016 cause even if it took 2 months, I didn't need a special area (that we just don't have).
