I'm working as a root so I should have permission to create any file

That is simply not true. There are a number of possibilities that can prevent even root to write to a file (or create it):

• file is readonly
• directory if readonly
• filesystem is readonly (for example CDROM)
• out of filehandles
• out of memory
• disk full
• file system errors that implicitly make the filesystem readonly
• file locked by another program
• directory locked by another program (most likely kernel during some directory move operations)
• file on a network filesystem where you don't have permissions
• Some "virtual" files can't be written to (/proc/*)
• ...The list goes on and on and on to ever more obscure "error" conditions...

BREW /very/strong/coffee HTTP/1.1
Host: goodmorning.example.com

418 I'm a teapot