NeoVim - UltiSnips Working, but Plugin Snippets Not Loading - plugins

Question

I've installed ultisnips using vim plug on Neovim. While the snippets engine works, the plugin installed snippets do not seem to work. The docs seem to suggest that writing 'let g:UltiSnipsSnippetDirectories=['UltiSnips']' will let me use plugin installed snippets (e.g., vim-snippets and vim-react-snippets), but this doesn't seem to work. How can I fix this?
Configuration:
set showmatch " show matching
set ignorecase " case insensitive
set mouse=v " middle-click paste with
set hlsearch " highlight search
set incsearch " incremental search
set tabstop=4 " number of columns occupied by a tab
set softtabstop=4 " see multiple spaces as tabstops so <BS> does the right thing
set expandtab " converts tabs to white space
set shiftwidth=4 " width for autoindents
set autoindent " indent a new line the same amount as the line just typed
set number " add line numbers
set wildmode=longest,list " get bash-like tab completions
set cc=80 " set an 80 column border for good coding style
filetype plugin indent on "allow auto-indenting depending on file type
syntax on " syntax highlighting
set mouse=a " enable mouseclick
set clipboard=unnamedplus " using system clipboard
filetype plugin on
set cursorline " highlight current cursorline
set ttyfast " Speed up scrolling in Vim
set backupcopy=yes " disable safe write
set ma
" set spell " enable spell check (may need to download language package)
" set noswapfile " disable creating swap file
" set backupdir=~/.cache/vim " Directory to store backup files.
call plug#begin('~/.vim/plugged')
Plug 'ryanoasis/vim-devicons'
Plug 'SirVer/ultisnips'
Plug 'honza/vim-snippets'
Plug 'scrooloose/nerdtree'
Plug 'neoclide/coc.nvim', {'branch': 'release'}
Plug 'mxw/vim-jsx'
Plug 'pangloss/vim-javascript'
Plug 'NLKNguyen/papercolor-theme'
Plug 'mlaursen/vim-react-snippets'
call plug#end()
let g:UltiSnipsExpandTrigger='<tab>'
" shortcut to go to next position
let g:UltiSnipsJumpForwardTrigger='<c-j>'
" shortcut to go to previous position
let g:UltiSnipsJumpBackwardTrigger='<c-k>'
let g:UltiSnipsSnippetDirectories=['UltiSnips']
:inoremap ii <Esc>
:nnoremap <F2> :NERDTreeToggle<CR>
colorscheme PaperColor
If relevant, I am using a macOS Monterey 12.3.