Thank you very much. I was on v4.1.8. Just updated to 4.3.0. I will re-try.

Make sure you are using the latest version. Switch -append is supported since 4.2.2 only.

Hello, I am for some reason unable to you -append switch with put command in a script

"C:\Program Files\WinSCP\winscp.com" /script=upload.txt

option batch continue
option confirm off
open xyz
put -append c:\hello.txt /NewYork/
exit

log file says
Unknown switch 'append'.