I got this error too, but in my case I was typing :pluginstall instead of the case-sensitive :PlugInstall and you have to use the correct case or it won't work. Oddly, :PlugUpdate was able to work before :PlugInstall but that was just because I typed the case correctly.
I see I have the vim.plug correctly installed here
~/.vim/autoload/plug.vim
I also created a "plugged" folder here and I see this is where vim-plug put the downloaded files from running :PlugInstall
.vim/plugged
To be overly pedantic, I typed 'cd' to get to the user directory, then cd .vim to go into my dotted .vim folder, then "mkdir plugged" to create the plugged directory. Not sure if this was 100% necessary or if :PlugInstall would have created it by itself.
I also needed to :source ~/.vimrc between adding addition plug line items and typing :PlugInstall, so make sure you "source" or you can also completely quit and restart your Vim so it will reload your .vimrc file, otherwise it won't know anything about your file edits.
I'm on a Mac. I installed vim-plug for standard vim and this gave me this file
~/.vim/autoload/plug.vim
My .vimrc is this:
" add line numbers
set number
call plug#begin()
" The default plugin directory will be as follows:
" - Vim (Linux/macOS): '~/.vim/plugged'
" Make sure you use single quotes
Plug 'tomasiser/vim-code-dark'
Plug 'tpope/vim-sensible'
call plug#end()