okay so the command, I think I made clear in the notes, is all on one line. It's not separated out.
additionally, you can't use the 'nuked laf' version of fastboot (that's a crippled version), you have to use the v35_engineering_abl version to flash that.
without seeing exactly what yer doing, that's all I can think of to help.
I followed the instructions to the letter and i approached with the easier way to give it a try
C:\Download>fastboot flash --disable-verity --disable-verification vbmeta_a blank_vbmeta.img
fastboot: unrecognised option `--disable-verity'