gvim Error Serverstart

Hello

want use the plugin on a mac with gvim (Version 8.2) I install the plugins with Pluginstall. No errors. I edit i file with with the name "test.md" IF i want call 'MarkdownPreview' the following Error appear

Error detected while processing function 15_serverStart:
line 4:
E117 Unkown function: mkdp#serverstart

my vimrc:

"Font in gvim
set guifont=Monaco:h20
"Zum Einschalten des Syntax-Higlighting
syntax on
"Zum Einschalten der Zeilennummern
set number
"Einstellen des Verhaltens von Einfügen und Löschen
set backspace=2 " make backspace work like most other programs

"automatische Installation des PluginManagers VimPlug
if empty(glob('~/.vim/autoload/plug.vim'))
silent !curl -fLo ~/.vim/autoload/plug.vim --create-dirs
\ https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
autocmd VimEnter * PlugInstall --sync | source $MYVIMRC
endif

" Specify a directory for plugins
call plug#begin('~/.vim/plugged')
Plug 'scrooloose/nerdtree', { 'on': 'NERDTreeToggle' }
Plug 'tpope/vim-fireplace', { 'for': 'clojure' }
Plug 'godlygeek/tabular'| Plug 'plasticboy/vim-markdown'
Plug 'iamcco/markdown-preview.nvim', { 'do': 'cd app & yarn install' }
Plug 'itspriddle/vim-marked'
call plug#end()

let g:mkdp_refresh_slow = 0
"let g:mkdp_path_to_chrome = '/Applications/Firefox.app'
let g:mkdp_path_to_chrome = '/Applications/Firefox.app/Contents/MacOS/firefox'

let g:mkdp_browserfunc = 'MKDP_browserfunc_default'
" Callback Vim function to open browser, the only parameter is the url to open
let g:mkdp_markdown_css = '/Users/uwe/github.css'

" für Markedvim
let g:marked_filetypes = ["markdown", "mkd", "ghmarkdown", "vimwiki"]