If you sign in the cydia impactor after inserting the IPA with you regular log-in it might give you an error code that will ask for an app specific password. If this happens go to https://appleid.apple.com/account/manage sign in and go to manage account, when you are there go to security and theirs an option to generate an app specific password, click on it, when you do so enter your normal password and it will generate a random password. Copy this password and go back to cydia impactor, insert the IPA again and login in with normal itunes email but instead of your normal password paste the password that was generated and it will then download.
