background color in vim colorscheme doesn't work properly in tmux

The background color is abnormal in tmux when using iTerm2

Try to reset the default settings of iTerm2; Minimum contrast

YouCompleteMe goes well in terminal vim but breaks in MacVim.

:echo $PATH in MacVim, see if it's different from which in terminal vim. If not, and you're using zsh, see $PATH in MacVim differs in terminal vim.

python error in MacVim

E837: This Vim cannot execute :py3 after using :python
E263: Sorry, this command is disabled, the Python library could not be loaded.