I used the command provided in https://github.com/ampproject/amphtml/blob/master/contributing/getting-started-e2e.md/CONTRIBUTING.md to install Node.js but it didn't work:

$ nvm install --lts
--lts.0.0
Node.js v--lts.0.0 is only available in 32-bit.

OS: Windows 10 64-bit

Workaround:

Run nvm list available to list available Node.js versions, then install the latest LTS version using nvm install x.y.z (8.11.4 in my case).