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. You should see your app listed. If it is not, then register a new app.

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

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

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

5. Enable Client Side OAuth Flow ==> checked

6. Disable Desktop Application OAuth Redirect Uri ==> not checked

7. OAuth Domain ==> stackexchange.com

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

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

10. 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.)

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. You should see your app listed. If it is not, then register a new app.

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

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

4. 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.

5. Enable Client Side OAuth Flow ==> checked

6. Disable Desktop Application OAuth Redirect Uri ==> not checked

7. OAuth Domain ==> stackexchange.com

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

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

10. 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.)