Cannot find module 'ts-node/register'

Since the answer that works for a lot of people appears to be hidden in the comments, I'll post it as an actual answer to the question, now that it appears the question has been reopened.

I had this problem as well. Not sure why this Q has been closed. but installing ts-node locally fixes this. npm install ts-node --save-dev

Thanks @Anita, as this was the answer that worked for me too.

Wow, a silly mistake can cost you time. I was facing the same issue when I was trying to debug my nodejs application. The mistake I had done was that I have created my .vscode folder outside of my nodejs app folder(the directory which had node_modules in it). When I moved my .vscode to that folder, everything work fine. Below is my launch.json file.

{
    "version": "0.2.0",
    "configurations": [
        {
            "type": "node",
            "request": "launch",
            "name": "index",         
            "args": [
                "src/index.ts"
            ],
            "runtimeArgs": [
                "-r",
                "ts-node/register"
            ],
            "cwd": "${workspaceFolder}",
            "protocol": "inspector",
            "internalConsoleOptions": "openOnSessionStart"           
        }
    ],
    "compounds": []
}

I know this is kind of old, but I ran into this too and wanted to offer the solution I'm currently using.

I installed ts-node globally using sudo npm i -g ts-node. To make this work with mocha, I just had to give mocha the absolute path to the module, like this:

mocha -r /usr/lib/node_modules/ts-node/register test/*Test.ts

Hope that helps someone else.