You need to find the original vendor_boot.img, vbmeta.img and boot.img files and flash them.
You can do it with MlgmXyysd's ROOT Toolbox.
Same question as Crackeds.
Thank you. This was on my mind but reading these posts confused me because of the discussions involving the vbmeta and the vendor images. I'm rooted now.
How to extract image collection from rom releases (and root the boot image and apply root to the phone)1. To extract the image collection from your downloaded rom : Download Payload Dumper from here (It's a zip file but it's not flashable) to your computer Extract the zip file to a folder, make it a folder you can use regularly because this is...forum.xda-developers.com
Try with this.