Are you saying you can delete existing files? Or that you have just uploaded a new one then deleted it again.
If the latter, and you cannot delete an existing file which you're not allowed to overwrite, I'd suspect permissions on the existing files don't match the user you're using for ftp. I'd be checking that.
I seem to recall a setting "AutoRename" which would not overwrite existing files it simply appends .1, .2, .3 to existing files. However, that wouldn't give you "permission denied".