Placing the Rocket Chat block on a page
install the livechat module and rocket_chat module.
  Setup the rocket_chat module.
  Go to [Structure][Block layout]. there you can place the livechat block 
  using the "Place block" button.
  This works as a normal block we recommend you add it to a footer or alike 
  for performance.
  


Source: Rocket chat module README.md

In other words, go to /admin/structure/block find the Footer region and click on Place block search for "RocketChat Livechat" and click Place block.