I've tried different ways and read some solutions including these:
Why I am not able to ssh as root?
Editing with Winscp
And an article from this Forum of
I don't understand why when I connect with ubuntu via putty, I can have root permission and modify files. However, when I connect with ubuntu via WinSCP, I'm another user(I used the same username and same private key(pem) on both connecting software) and can't modify files.
Then I found some possible solutions from the links above. I tried to set root password and permitRootLogin. But then, when I log in with the root username, it says the file is too big, whereas when I changed the username back to ubuntu, then it works(file is size has no problem, but I can't modify files when I'm not root user).
I am very confused, please anybody if you can point me out, I'd be much appreciated.