I couldn't initially find the render controls, so I tried switching engines and it didn't work):
You will find crowdrender says it is rendering and the render buttons will never appear. Instead, leave it as your preferred engine (e.g. Cycles). This is because Crowdrender adds a section under each engine's properties panel and then auto-switches to it's own engine when you press one of the Render buttons (it will switch back when done).
If you can't find the Crowdrender section, it's probably at the bottom of Render properties panel. Drag it up with the handles to make it easier to find:
If someone could update the documentation to explicitly inform the user to not switch rendering engines, it might help eliminate confusion. :)