Remote Erased A7 2017 (A720F) that had SuperSU. Apparently unrooted but still cannot be updated.

I ran remote erased on the phone because I thought I had lost it, but had later found it. I downloaded the SuperSU app to see if its still rooted, and it is not. So I ran an update check, and it throws an error saying ‘The operating system on your device has been modified in an unauthorized way. Try downloading software updates using Smart Switch on your computer or visit a customer service center.’ I tried updating with Smart Switch, but it fails with an ‘unknown error’. Its last update was August 3, 2017 and TWRP 3.1.0-1_ashyx is still installed. I would very much prefer to get Android 8 on this phone.