Failed to start hostapd.service: Unit hostapd.service is masked

[gulliverrr]

This is obviously not your problem but raspbian Stretch's hostapd as it changed their installation steps and a little edit is needed in your installation steps here under README.
Run these:

sudo systemctl unmask hostapd
sudo systemctl enable hostapd

before running this:

sudo systemctl start hostapd && sudo systemctl start dnsmasq

Here is a little background reading.

[MushegKh]

This has solved my not working AP in RPi3.
SSID was not visible.

[jose91sdr]

Great, it solved the same problem on my Raspberry Pi Zero W, thanks