Can't import custom Toolbox in ArcPy?

Question

I have a custom made Toolbox and I can't make it run.

The Toolbox is called toolbox_k and the tool is called sn

I did this:

arcpy.ImportToolbox(r"C:\Users\user\Desktop\VST\Toolbox_k.tbx", "sn")

I use the sn with the parameters as using the GUI version and it doesn't work. It gives a syntax error:

sn('pathone','pathtwo') 

UPDATE

Running:

>>> arcpy.ImportToolbox("Toolbox_PS.tbx", "tbxk")

gives:

Runtime error 
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "c:\program files (x86)\arcgis\desktop10.6\arcpy\arcpy\__init__.py", line 125, in ImportToolbox
    return import_toolbox(input_file, module_name)
  File "c:\program files (x86)\arcgis\desktop10.6\arcpy\arcpy\toolbox_code.py", line 457, in import_toolbox
    mymodule = generate_toolbox_module(toolbox, None, False, False, False, module_name, use_alt_alias)
  File "c:\program files (x86)\arcgis\desktop10.6\arcpy\arcpy\toolbox_code.py", line 432, in generate_toolbox_module
    'exec')
  File "C:\Users\user\Desktop\VST\Toolbox_k.tbx", line 19
    def 00CalculateKAEK(PST=None):
         ^
SyntaxError: invalid syntax

Answer 1

From the documentation:

ImportToolbox (input_file, {module_name})

• input_file - The geoprocessing toolbox to be accessed from Python.
• module_name (optional) - If the toolbox does not have an alias, the module_name is required.

When a tool is accessed through the ArcPy site package, the toolbox alias where the tool is contained is a required suffix (arcpy._ or arcpy..). Since ArcPy depends on toolbox aliases to access and execute the correct tool, aliases are extremely important when importing custom toolboxes. A good practice is to always define a custom toolbox's alias; however, if the toolbox alias is not defined, a temporary alias can be set as the second parameter.

So try:

# *If* you have set an alias in your .tbx properties, 
# assuming you set the alias as "tbxk"
arcpy.ImportToolbox(r"C:\Users\user\Desktop\VST\Toolbox_k.tbx")

# If you haven't set an alias, use a temporary alias "tbxk"
arcpy.ImportToolbox(r"C:\Users\user\Desktop\VST\Toolbox_k.tbx", "tbxk")

# then
arcpy.sn_tbxk(etc...)
# or
arcpy.tbxk.sn(etc...)

Re. your update with theSyntaxError - python names can contain numbers, but must start with a letter or an underscore so you'll have to rename def 00CalculateKAEK(PST=None)