link2sd – Fix for error : “mount: ‘/system’ not in /proc/mounts”

First things first, I just got into rooting and custom roms. I haven’t written a single command, and I am just at the start of a beautiful journey. I am going to try to include everything, since I don’t know which part is important.

I have a redmi note 5 pro, was android 9, at that point I unlocked the bootloader and rooted it with magisk. Didn’t download any module.

After that I donwloaded a custom rom (pixel experience) (I use pitch black recovery). The device got unrooted and I had to root it again via magisk. I wanted to root because there were certain apps that I wanted to convert to system apps. I tried downloading a module called App Systemizer through magisk and tried to run it, but to no avail, maybe because I am running android 10. After that I searched for alernative ways, and found an app Link2SD . I tried to convert an app to system app, and got shown this error :
Error in shown by Link2SD

So what do I do now? I am looking for permanent solutions. I have tried looking up other answers and most of them were like “… write this code…” But where am I supposed to write and implement these codes? Also, I somewhere read that maybe my /system is read only, so then how do I convert to read-write mode? Is there way to solve it without compromising my data (I know I can take backup, but still).

All help is appreciated .