It looks like you're having an issue following a tutorial hosted on another site, product or service called App Inventor. This forum is for developers and users of the Google App Engine platform to discuss it.
It seems as though the App Inventor example you were attempting (which, from your post I can tell involves something called "tiny web db") was hosted at
aarhus8210h.appspot.com. It appears that the web app deployed at this subdomain doesn't have a request-handler which is serving an HTML page or file at "/".
If this isn't an app you have created, you won't have any control over whether it serves or not, so I suggest contacting the creator of the tutorial you were following to let them know the site at the given link is down.
Was
this the tutorial you were following?
I wish you the best of luck in contacting the maintainer of the tutorial you were following. As it stands, this issue isn't related to App Engine as a platform, but rather concerns only the person or organization which is meant to have deployed an app at
aarhus8210h.appspot.com.