install sites using virtual but ftp upload failed

I used virtualmin to creat multi site on a vps.
everything is OK except when I try to upload and try to update a file(former files on the vps are scp from another vps)
it always reports

550 somepage.html: Permission denied
Error: Critical file transfer error
Status: Disconnected from server

your comment welcome

even public_html set to chmod 777

when I try to set the a file
ftp reports

SITE CHMOD 777 somepage.html
Response: 550 CHMOD 777 somepage.html: Operation not permitted