This is now sorted after disabling Bluetooth and ensuring that “enable_uart=1” is set in /boot/config.txt
After I disabled Bluetooth, the interface /dev/ttyS0 or /dev/serial0, showed up.
I set the input on MH-Z19B to use /dev/ttyS0 as per: Mushroom Cultivation Automation: From Foraging to Fruiting – Projects | Kyle Gabriel and boom it started picking up readings.
References
Similar issue: MH-Z19B Reading problem - #11 by KyleGabriel & MH-Z19B: CO2 Sensor Error · Issue #1162 · kizniche/Mycodo · GitHub
How to disable Bluetooth: Tips - Disabling Bluetooth on Raspberry Pi
Mycodo manual: https://kizniche.github.io/Mycodo/mycodo-manual.pdf#Interfaces/