Please post questions about MML16CN here: https://customercare.velleman.eu/
Considering this is not one of the “Velleman For Makers” products.
You can try to reset the MML16CN, due removing the Intern battery CR2032, and placed back after 2 minutes (or replace battery if empty). Check also if the CPU is properly seated in the CPU holder.
If this doesn’t help, something wrong with mainboard. If product is still under warranty (2 Years) you can bring it back were you purchased from it.
For sending messages, first you need to assign a ID address. if not you’ll get indeed this error message. Click on Functions => Set ID give in as ID 00 (address 0).
Then Type a message, and send it as address 00 .
Click **Functions => Send Message => give in as ID 00 and page A => click ok.
Message will be shown on display without error, a beep sound is emit.