I don’t have the extension, but I think I found something in the forums.
Did you upgrade the firmware to 2.11 and tell the controller that the extension is installed?
According to the firmware upgrade notes:
Added support to Z-Axis Extension Module. When you attach the Z-Axis Extension Module to your printer, upgrade the firmware to [version 2.11]. Reconnect Snapmakerjs to the printer. Enter “M1025 M1” in the console and press “Enter”. When the software replies “ok”, your printer is upgraded successfully.
When you want to use the original linear module, enter “M1025 M0” in the console and press “Enter”. When the software replies “ok”, your printer can only print items as tall as 125 mm.
Also, you need to upgrade your software to the latest version and change the settings of the machine to generate G-codes for taller models.
The article has links to the new firmware and software.