[OnePlus 6 Questions & Answers] zobayer.a.m:
Actually I follow below threads to flas manually. Not only 2 lines, I wrote all commands. Just those 2 commands were not executed properly.

https://xdaforums.com/oneplus-6/how-to/rom-stock-fastboot-roms-oneplus-6-t3796665/amp/

I also try to flash by clicking flash all.bat. At that time my windows is in test mood and all necessary tools are installed. It execute properly, but again I stuck into boot loader. Here is log in CMD

---------------------------------

extracting android-info.txt (0 MB) to RAM...

extracting boot.img (64 MB) to disk... took 0.259s

target reported max download size of 805306368 bytes

archive does not contain 'boot.sig'

archive does not contain 'boot_other.img'

extracting dtbo.img (8 MB) to disk... took 0.037s

archive does not contain 'dtbo.sig'

archive does not contain 'dt.img'

archive does not contain 'recovery.img'

extracting system.img (2860 MB) to disk... took 22.194s

Invalid sparse file format at header magic

archive does not contain 'system.sig'

archive does not contain 'system_other.img'

extracting vbmeta.img (0 MB) to disk... took 0.002s

archive does not contain 'vbmeta.sig'

extracting vendor.img (1024 MB) to disk... took 10.539s

Invalid sparse file format at header magic

archive does not contain 'vendor.sig'

archive does not contain 'vendor_other.img'

wiping userdata...

mke2fs 1.43.3 (04-Sep-2016)

Creating filesystem with 28835840 4k blocks and 7223040 inodes

Filesystem UUID: 6d3b9e4c-9abd-11ea-82c2-c3024ecf0407

Superblock backups stored on blocks:

32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,

4096000, 7962624, 11239424, 20480000, 23887872

Allocating group tables: done

Writing inode tables: done

Creating journal (131072 blocks): done

Writing superblocks and filesystem accounting information: done

--------------------------------------------

Bootloader Version...:

Baseband Version.....:

Serial Number........: 2fb33b77

--------------------------------------------

sending 'boot_a' (65536 KB)...

OKAY [ 1.964s]

writing 'boot_a'...

OKAY [ 0.301s]

sending 'dtbo' (8192 KB)...

OKAY [ 0.238s]

writing 'dtbo'...

OKAY [ 0.108s]

erasing 'system_a'...

OKAY [ 0.151s]

sending sparse 'system_a' 1/4 (780553 KB)...

OKAY [ 18.710s]

writing 'system_a' 1/4...

OKAY [ 0.016s]

sending sparse 'system_a' 2/4 (762213 KB)...

OKAY [ 20.487s]

writing 'system_a' 2/4...

OKAY [ 0.013s]

sending sparse 'system_a' 3/4 (786133 KB)...

OKAY [ 21.143s]

writing 'system_a' 3/4...

OKAY [ 0.014s]

sending sparse 'system_a' 4/4 (395961 KB)...

OKAY [ 12.300s]

writing 'system_a' 4/4...

OKAY [ 0.004s]

sending 'vbmeta' (8 KB)...

OKAY [ 13.161s]

writing 'vbmeta'...

OKAY [ 0.011s]

sending sparse 'vendor' 1/2 (757755 KB)...

OKAY [ 18.524s]

writing 'vendor' 1/2...

OKAY [ 0.011s]

sending sparse 'vendor' 2/2 (114716 KB)...

OKAY [ 7.214s]

writing 'vendor' 2/2...

OKAY [ 0.012s]

Setting current slot to 'a'...

OKAY [ 10.843s]

erasing 'userdata'...

OKAY [ 0.673s]

sending 'userdata' (5273 KB)...

OKAY [ 0.143s]

writing 'userdata'...

OKAY [ 0.008s]

rebooting...

finished. total time: 126.273s

Press any key to exit...

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