1, You need to make the toolhead back to its home position so that it can move down on the Z-axis. On the touchscreen, go to Controls > Home Axes to home the toolhead.

2, Make sure all the cables, especially the Z-axis, are properly and correctly connected to the controller according to the Quick Start Guide or the provided Connection Card.

3, On the Touch Screen, go to Controls > Jog Mode. Select Move 10 mm and then tap Z-, Z+. If the 3D Printing Module moves according to your operations, there is nothing wrong with your printer. If the 3D Printing Module doesn't move according to your operations, please go to step 4.

4, Please upgrade the firmware to the latest version. Here I attach the zip compression package and you can also download the file via this link https://forum.snapmaker.com/t/downloads-and-updates-for-firmware/1467

5, Please check the limit switch of this linear module if it is too tight or not. Here is another image to show you the screws. If the screws are too tight, the linear module may be influenced.

6, You can switch the Z-axis with another linear module to see if the new Z-axis works or not. For example, you can switch the ports of the Y-axis and the Z-axis. 
7, Please disassemble the Z-axis and put it away from the baseboard. Here I attach an image for your reference.

Power on the controller and control the Z-axis, and observe if it can move smoothly in both directions.
If it does move in two directions successfully, the problem lies in the way you assemble them. Please switch the X-axis and the Z-axis because the limit switch cannot get to the ground when it is working.

8, Turn off the printer, unplug the cable from port 5 and plug it into port 4, and then turn on the printer. Repeat step 2. If the 3D Printing Module moves according to your operations, there is something wrong with port 5. Please use the printer with port 4. If the 3D Printing Module doesn't move according to your operations, please contact support@snapmaker.com for assistance. You need to have a new linear module or a new controller. Please check if your machine is under the limited warranty. If so, we will replace the parts for you. Or you can buy a spare part from us directly.