May be you already solved the problem, but to help another people to save time I tell my workaround.
It seems that digi image doesn't contain sftp-server that required by sftp client.
If you type:
then you get:
sh: /usr/lib/openssh/sftp-server: not found
To avoid this issue:
1. switch to Projects mode (Ctrl-4)
2. for board kit choose Run tab
3. in Deployment settings disable step "Upload files via SFTP"
4. push the button "Add Deploy Step"
5. choose "Custom Process Step"
6. write in Command: scp
7. write in Arguments: local_file_path root@digi_board_ip:remote_directory
local_file_path and remote_directory are the same as in table "Files to deploy" in the same Deployment settings.