If you want to copy text to the clipboard in Neovim you can do so with xclip. The command is a bit complicated so I highly recommend you binding it to a shortcut.
This is what I have in my Neovim configuration to let me copy marked text using
vmap <C-c> y:call system("xclip -i -selection clipboard", getreg("\""))<CR>:call system("xclip -i", getreg("\""))<CR>
Just make sure to install the tool
xclip if you don’t already have it.