Have you ever got an error when using hostapd complaining a network interface not be found but its actually there and ready? You probably have a space at the end of the line “interface”. Hostapd does not work when having a space in that line (and probably in other lines as well) in /etc/hostapd/hostapd.conf.
ap:/etc/hostapd# cat -vet hostapd.conf|grep ^interface interface=wlp0s29f7u7 $ ap:/etc/hostapd#
This shows that there is a space at the lines end. Remove it and it will work as expected. Remove it and it works. If it does not work you probably have an error in another layer of your WLAN setup.