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.
Enable Client Side OAuth Flow ==> checked
Disable Desktop Application OAuth Redirect Uri ==> not checked
OAuth Domain ==> stackexchange.com
Application Name ==> (not blank and no error message)
Description ==> (not blank and no error message)
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.)