Heroku has a set of default buildpacks, used when it needs to detect the language of your app.
In order to do that detection, it runs the
bin/detect command of each of those default buildpacks, until one of them returns a 0 exit code.
This is the command for the node buildpack.
As you can see, it requires a
package.json to be located at the root of your app, not in a subfolder.
This is the difference causing your build to fail. You need to put your app at the root of your git repository.