Hi,
As you probably know, there are Chinese (A) and Globals(G) versions of Flyme.
Probably the old user used an old method to install the Global version 5.1.2.0G. With this old method, he could install this specific version but in fact, the phone is not globalized(You can't install G versions, but you can install A versions).

If you need to install Global versions (G), please with a root explorer application like "Root explorer" or "Root browser" open as text the file /dev/block/platform/155700000.ufs/by-name/proinfo and post it here. The content of this file could be "machine_type=M576_unicom_custom", "machine_type=M576_mobile_public" or "machine_type=M576_intl_official". Only if the value is "machine_type=M576_intl_official" you can install Global versions.

Regards.

You can globalize your phone changing the text in the file /dev/block/platform/155700000.ufs/by-name/proinfo to "machine_type=M576_intl_official"  (Only can be done in some flyme versions. I have succed changint it in 5.1.2.0A so I suppose you could change in 5.1.2.0G without problem) and now you can install G versions.

Hi, as I told you, you have a chinese model so you can only update with A versions.
For root you have to use the supersu 2.79 or older to avoid the " binaries are occupied" issue. You can download it here:
https://www.apkmirror.com/apk/co ... droid-apk-download/
Hope this helps.

Other note: For changing the proinfo (for globalize your phone) you need the version 5.1.2.0A (it is probably not going to work in 6.1.0.0A).
Regards,