Upgrade failed due to a failure in downloading the version file

Hello,
Advised  you to test using SFTP and run the ssh client first-time enable.

Is there a problem with the connection between your server and the AP? Check it. 

your AP same as mine AP5030DN
I encounter that version error when updating upgrade-assistant-package
in my case I need to run "upgrade version check" first
]upgrade version check
Info: Upgrade version check ok.
]ap-mode-switch check
Info: Ap-mode-switch check ok.
] diagnose
diagnose]upgrade upgrade-assistant-package ftp FatAP5X30XN_V200R019C00SPC907.upgrade_assistant_package.bin 192.168.88.1 admin admin
if you got "Error: Upgrade upgrade-assistant-package failed due to a failure in downloading the version file."
that mean ftp down, try ftp manually from your pc or use sftp instead like this
enable sftp using coreftp lite edition then use this
-diagnose]upgrade upgrade-assistant-package sftp FatAP5X30XN_V200R019C
after that I can upgrade FAT normally
]upgrade version ftp FatAP5X30XN_V200R019C00SPC907.bin 192.168.88.1 admin admin