404 means that the location you entered cannot be found.
In the Android App in the Kegbot URL did you enter the URL of your Kegbot Server? It sounds like you’re running on a different port number so the format of the Kegbot URL would be http://192.168.1.1:3000 or http://mykegboturl:3000. Replace the IP Address or mykegboturl and port number with your actual IP address and port number of the Kegbot Server running on your Raspberry Pi.
It sounds like you may have already gotten past that part if you’re making it to the Link This Device page in the App. Once there, visit the website for your Kegbot Server and just add ‘/link’ to the end of the url (eg. http://192.168.1.1:3000/link). This should give you the opportunity to enter the code displayed on your android tablet and once complete the tablet should automatically cycle to the next screen and will be fully linked.