There might be different reasons for this. Check if you situation falls under any of the categories below.

You changed package.json without committing and pushing the updated package.lock (or yarn.lock)

In this case, you'll likely see the following error in the build logs:

We use npm in all our challenges so the solution would be to delete yarn.lock if you have it, run npm install locally, and commit and push the updated package.lock file.

You used a dependency that's installed globally on your local machine

In this case, make sure to add this dependency to your project.

You changed the test script in package.json and the build is now hanging

E.g. you did something like this.

"test": "./init_your_app & cypress run"

This will likely cause a hanging build.

To solve this, undo your changes in the test script ("test": "cypress run") and add any commands that are needed to initialize your app in the build script (e.g., "build": "./init_your_app").