Here is a guide that I found that will walk you through the steps to make your Raspberry Pi turn itself into a hotspot when it can not detect a WiFi network.
It takes less than a minute for the auto hotspot to show up after boot up as the Pi searches for a recognized network first.NOTE: May not work on all RPi3's.
RPi3 Auto Hot Spot (all credit goes to Roboberry)
When you get to this part of the article........USE THE DOWNLOAD if you are copying and pasting the code into your Pi's terminal.
I think there is something missing in the article and it will cause the Auto Hotspot to fail.