I suspect you are trying this under LineageOS 17.1 which uses Android 10. AFAIK, Nexmon is not compatible with Android 10. I have a 64 GB model, and monitor mode works fine without needing Nexmon. Open the main NetHunter app, and go to Custom Commands section. Find the item titled “Start Wlan0 monitor mode”, and the Run button to the right of it.
Boom, your internal WiFi is now in monitor mode. To exit monitor mode, tap run next to “Stop Wlan0 monitor mode”.