root access failed or busybox was not found Phone: Xiaomi Redmi 4 prime
Rom (custom): lineage-13.0
Xposed: xposed-v87-sdk23-arm64
Root: SuperSU.Pro.2.82


Right now everything works just fine and those apps that need root privileges to work properly, asking for root permission and after granting, they work fine;

There's just one app call Performance Profile (an xposed module) that after running asserts that: "root access failed or busybox was not found"

I do have successfully installed BusyBox 6.7.3.0

What is the problem with this app?

Update 1: I uninstalled BusyBox 6.7.3.0 and installed BusyBox 1.26.2 and using BusyBox Checker Pro 2.0 got the following results showing that everything is ok.
A wild guess: You did not correctly install busybox.

BusyBox is not a simple application, it's a executable binary that should supposedly be located in /system/xbin. A correct installation of BusyBox includes copying the binary and creating symbolic links.

Judging by the version number you gave, you might probably installed something like "BusyBox installer" from Google Play or so. The (currently) newest BusyBox binary version is 1.26.1, so you may want to check again if BusyBox is installed correctly, not an app from GP.