DownGrade Android 9 to 8.1

[longthk19]

Please tell us some basic information before asking for help:
Model Name: ZB570TL
Firmware Version: Version WW-16.02.1905.435
Rooted or not: not
Hi, I downgrade like your instruction. www asus com/support/FAQ/1040827/
(space= . )
But it is stucked at Step 6 with error :
Support API: 3
::Failed to mount /sdcard: Invalid argument
Couldn't mount /sdcard/
Installation aborted.

[Christine_ASUS]

Hi longthk19
Thank you for your asking. Kindly provide the information below so that we could better assist you.
1. Could you try with another MicroSD card? Make sure that your MicroSD card is clean or doesn't have any storage issue. To avoid any incomplete downloads, please re-download "Version Android P downgrade to O" via PC.
#ZB570TL Firmware Spot
https://www.asus.com/Phone/ZenFone-Max-Plus-M1/HelpDesk_BIOS/
2. Make sure that the download file is put into the root directory of the MicroSD card. (The most outside place you could put into the MicroSD card.)
If the situation remain the same, could you provide me a video about how you downgrade your phone with MicroSD card?