Alright, I'm going to tell you what I'm doing step by step then you tell me what I'm doing wrong. Starting from scratch.
1. Flash stock room
2. Flash twrp
3. Wipe data
4. Reboot to bootloader
5. Run command "fastboot wipe userdata"
6. Flash twrp again
Is this correct?
Lemme redo it for you.
1. Flash stock rom (don't restore backups, probably you removed encryption from it)
2. Flash twrp (I hope you won't be facing decrypt issues)
3. Wipe data
4. Flash lineage-15.1 & arm64 gapps 8.1
5. Reboot system