1 Check if your device can spawn multiple AP

iw phy0 info

# ....
#         valid interface combinations:
#                  * #{ managed } <= 1, #{ P2P-device } <= 1, #{ P2P-client, P2P-GO } <= 1,
#                    total <= 3, #channels <= 2
#                  * #{ managed } <= 1, #{ AP } <= 1, #{ P2P-client } <= 1, #{ P2P-device } <= 1,
#                    total <= 4, #channels <= 1

• #{ AP } <= 1 means it can only spawn 1 access point.

If you are running a wireless headless pi, you cant spawn another AP, since one is already in use to connect to the Pi.

Instructions for hostapd is in /usr/share/doc/hostapd/README.Debian
(Cool tidbit: you can just use linux file path into chrome browser url search bar to read the text)

hostapd loads configs from /etc/hostapd/hostapd.conf

Copy the example configs

sudo cp /usr/share/doc/hostapd/examples/hostapd.conf /etc/hostapd/hostapd.conf

if need be, look into your hostapd.conf and make sure it uses a valid interface, sometimes it your PC may use conventional naming like wlan0 which your can check with ip addr

systemctl unmask hostapd
systemctl enable hostapd
systemctl start hostapd

2 Properly checking if hostapd failed and how

/usr/sbin/hostapd /etc/hostapd/hostapd.conf -dd | tee /tmp/hostapd.log