SOLVED: "Cannot mount /vendor" :MAGISK

This post is a help to Fix Magisk Install Error 1 problem in TWRP.

Okay, so you might be either switching from Super Su to Magisk, or this might be the first time you're rooting your phone, but one this is for sure: You are Rooting your phone.
Let's get into this, What is Magisk?

Magisk is an alternative to Chainfire's SuperSU, a better alternative to be precise. With Magisk you can hide your rooting. This is helpful specially when you want to use Banking Apps Like: Google Pay,PayTM, PayPal or even some games like PokemonGo which detect your device is rooted and thus does'nt work normally or doesn't work at all. For this Problem, Magisk is for your rescue. It systemless-ly masks your device so that these Apps cannot detect the Root.
Magisk is created by Topjohnwu, who created Magisk Manager and kept is open Source unlike SuperSU. Thus, Magisk is more likely to be improved by any developer who wish to contribute.

Let's move on to Rooting your device:

Rooting  your device is simple and easy, though it has some risk of bricking your phone. I recommend only developers to root their phone.
I won't be going into the same process depicted by other sites and only to flash and result in Error1.
I've gone through many process like:

• Using a different ROM.
• Formatting your phone completely.
• Extracting your ROM's .zip file and Flashing it.
• Extracting .img boot file, pasting it someplace in internal Memory.

All the above are crap, just time wasting and data wasting.

Let's to the Step by Step Procedure:
NOTE: I'm a Redmi 4A developer and tester, so this might not possibly work for other phone model. but I've tester for Redmi 5A and Note 4 as well.

1.  Full charge your Phone. This is a necessary step a few don't bother considering it and have faced a lot of issues.
   
   
2. Take a complete Backup of your Phone.
   
   
3. Go back to Stock ROM(The ROM that came pre-installed on your phone). If you are first time rooting, just ignore this step.[Not a Mandatory step. works on most custom ROM as well. I've tried AEX, LineageOS15.1, RR6.0]
   
   
4. I've never seen Magisk didn't work on  any ROM, so don't worry, I got you. Note: All these ROM had Oreo.
   
   
5. Make sure you've unlocked your bootloader. Now download TWRP but here's the thing. Download TWRP recovery with vendor. Download link: REDMI_4A_TWRP- Rolex
   Redmi_5A_TWRP- Riva              
   
   
6. Now install this Recovery using Fastboot, using PC, I recommend Minimal ADB & Fastboot.
   
   
   
   
   
   
7. That was it. Now flash Magisk and Reboot to system.
   
   
8. You will an App Magisk Manager installed on your device.
   
9. That's it. you're done. Now to hide Rooting, this manager will help to install a file and select the apps from which you wish to hide the Rooting. Enjoy!!
   

Further to install any Custom ROM. Flash the zip file, your Magisk would still be installed. and phone would still be rooted.😎

Further information by the developers:






For further queries, comment down. Hope to help you all. 👍