Go to your cPanel --> FTP Manager --> FTP Accounts:
Make a FTP account for subdomainusername (the same as the folder created by the subdomain).
The FTP client server field will be either your domain or the server IP number.
The username will be
folder
main.domain and the password will be the one given when you made the FTP account.
The user will now have read and write privileges to that folder and all folders below.