Not able to open web browser to local host

Hello. I am new to Docker. I am currently following the tutorial to build a Sample Application, and have reached step 2. This is the URL: Sample application | Docker Documentation

I am currently stuck on “Start an app container”, where I have to put this in the command of a terminal:

docker run -dp 3000:3000 getting-started

After typing this in the command I have to go to a site on a web browser called: http://localhost:3000

When going to the site, I should see an app.
However, when I open the site, I don’t see an app. Instead I see:

This page isn’t working
localhost didn’t send any data.

I tried running docker run -dp 3000:3000 getting-started in the command repeatedly after this, however it said the port is already allocated,

How can I get the page to work so that I can continue with the tutorial? Thank you.

A host port can not be bound more than once. You need to find which application or container on your host already uses that port OR use a different host port, e.g. docker run -dp 3001:3000 getting-started and then use http://localhost:3001 to access it.