As my earlier post "Checking foe a successful root" does not seem to have found a definite answer, I'm assuming that the flashing has not resulted in a rooted Android.
Oddly my very first attempt at a rooted Android did in fact work. I'd flashed it to have a rooted Android and Sailfish OS. It did work, but I've no idea how or why. I didn't like Sailfish over much, and decided to replace it with the Debian alternative. and re-flashed the thing, and replaced Sailfish with Debian. However, to my mind the full monitor screen layout did not suit the small display in a workable fashion. I also found out that the option to invoke root access was absent. The machine reported that there was no executable command 'su' and listing was not available for anything at root level.
Since then I've re-flashed the thing more than a dozen times in every combination possible, but every time I get the same answer. The final time, I opted for a double boot of Android. Boot one for normal and boot2 for rooted. It did work, I could boot into either, but neither of them appears to be a rooted Android.
Since the whole operation is carried out on a "monkey see, monkey do" fashion, there is little one can infer to explain it. It does seem a little odd that both the normal and the rooted option use the same zipped up set of image files, the loaded image for both should have the same complement of files, including the su executable. If anyone knows how it all works, I'd be most interested. The "monkey see, monkey do" principle infuriated me for the whole of my time in IT admin, since for what seemed about 80% of the time, the given instructions failed to perform, leaving you in a sort of no-mans land, with no idea of where to go from there. Whenever we finally got a piece of kit working, we would vow never to touch it again, since it was seldom that we knew the how and why of our final success.