This means Vim is trying to load the function plug#begin() then the
function plug#end() from the script <something>/autoload/plug.vim
where <something> is any one of the directories mentioned in
'runtimepath', and either not finding that script, or not finding
these functions within it. It also doesn't find the user-command Plug
so the former (script not found) seems more likely.
Try adding the following at the top of your vimrc, it should give you
a warning message for not finding files to be sourced:
if &verbose < 2
set verbose=2
endif
Later you can manually :set vbs-=1 if you want to stop displaying all
those messages.
See :help autoload
Best regards,
Tony.