That error means that you've used an invalid id or that the settings on your app page are incorrect.

Do the following:

 1. Go to [*Your Apps* page][1].  You should see your app listed.  If it is not, then [register a new app][2].

 2. Click on your app's settings page. For example: [stackapps.com/apps/oauth/view/3996][3].  Note that only the owner can see a given app's settings page.

 3. For [the kind of authorization][4] you seem to be attempting, make your settings as follows:

   1. **Stack Apps Post** must be set to a valid post that you own.  
     In this case, `https://stackapps.com/questions/5017/` should work for *user2621858*.

   1. **Enable Client Side OAuth Flow** ==> checked

   1. **Disable Desktop Application OAuth Redirect Uri** ==> not checked

   1. **OAuth Domain** ==> `stackexchange.com`

   1. **Application Name** ==> (not blank and no error message)

   1. **Description** ==> (not blank and no error message)

   1. **Application Website** ==> `http://localhost/foo/` (Not critical for this kind of auth.)


----------
Finally, be sure that your app hasn't been disabled for gross attempts at violating quota limits or terms of service or ignoring the `backoff` flag. (I'm not sure what the error message would be for a banned app and that is probably not the case for this OP at this time.)

  [1]: https://stackapps.com/apps/oauth/
  [2]: https://stackapps.com/apps/oauth/register
  [3]: https://stackapps.com/apps/oauth/view/3996
  [4]: https://api.stackexchange.com/docs/authentication