You can use reprex package on Rstudio Cloud, you just can't use it with the terminal clipboard content, but you can manually paste the code into the function call.
Even if xclip was installed, you wouldn't have access to the terminal clipboard, because there is no ssh connection with X11 forwarding between the terminal and the server, or at least I don't know how to configure this, do you know how to do it? If you do, please share with us, it would be awesome to use reprex with the clipboard on Rstudio server.