I would like to assign operation to a command key in Vim. Aren\'t any methods?
Supposing you are on a Mac, you can't map the command key in vim, only in MacVim.
If you are using MacVim, your mappings should use <D-…> as in nnoremap <D-l> <C-w>l. But it's not a very good idea for many reasons among them those listed in this other answer of mine.
Use <leader>, as in nnoremap <leader>l <C-w>l, instead.
See :help leader and :help key-notation.