Test your app extensively and fix the crash. Enter wrong values, don't enter anything at all, intentionally try to break your app and get it to crash. Then you will find your error and you can fix.
EDIT BASED ON COMMENT BELOW
"My app don`t need conect with external database, because I got the information inside" - in this case, no, you do not need to add IPv6 support because you are not communicating with the internet for logging in purposes.
You can keep the login given it is only going to be 2 people using the app, just resolve the crash when logging in.
To test, enter a variety of incorrect passwords and usernames to ensure that there is no crash on incorrect entries, and confirm that correct entries do not crash the app.
Original Answer
Apple indicated that because there is a login feature, they think it is trying to communicate with an external database (ie a database that is not on the device). If there is no external database, just fix the crash is what they are asking.
Based on your question, you have eluded to the app not communicating externally for logging in. While some may find this odd, it is not uncommon. An example for those wondering would be a childrens app for iPad. The iPad might be shared amongst several children and as such each child might complete different sections or features of the app so an account local to the device is a good idea if there is no cloud support.
If there is external communication, you need to handle the case for no internet access. The question has been answered extensively, however this was the first result I found:
Detect Internet Connection and display UIAlertview Swift 3
Update: Why do I need to Support the case for no internet connection?
A use case:
John has just downloaded your app. He has just walked through the steps to create an account, but he has accidentally set his device to Aeroplane Mode.
John hits the "Create Now" button but nothing is happening. There is no error or success alert appearing, the screen has not changed, he can't see a loading icon. John is confused and getting frustrated because he's certain that he has completed everything in the form.
John doesn't understand that he needs internet connectivity to successfully submit a request to create an account. John decides to delete your app from his device and leaves a bad review.
John really could have used an alert saying "You need internet connection to sign up to this app. Please check you are connected and try again". This would have made John a whole lot happier and he probably would have continued to use your app with all of it's amazing features.
More information can be found in the guidelines that Apple mentioned, and further to this, the Apple Design Principles Guide is an invaluable resource.
I hope this makes sense.
