You cannot use "CURRENT" in a Geoprocessing Service. The GP Service has no idea what "CURRENT" is as the service isn't an open session of ArcMap. CURRENT is reserved for working with a script inside ArcMap only.
To use arcpy.Mapping, specifically a reference to a map document, you need to provide the full path to the MXD in place of CURRENT.
mxd = arcpy....
I think you are close...however
Based on this documentation, ListWorkspaces returns a list. So to use the output,
First call to get the Workspaces list
Then check the length
If the length is > 0 then get the first element which is zero-indexed 
Use that element of the list as a string in the concatenation
Sample code (not tested)
datain = arcpy....
I did it in an other way:
using database consist of fivel levels,
without choosing the shapefile or fields
just by selecting items from first level
the validation script generates the values for the second level according to your choice in first level, her the script:
"""Class for validating a tool's parameter ...
My answer is similar to Mani's. I also have a complicated screen setup (docked laptop, logged into a Virtual Machine, viewed on separate dual monitors) and I believe the Item Description Window is actually opening, just not within the view-able screen space.
An old post on GeoNet explains the fix for ArcMap 10.0 back in 2010: https://community.esri.com/...
The answer is here! I am also having the same problem and there is an official answer.
In short, in the 'value fields' choose only ONE variable, in the 'variation by-symbol size' choose another variable
In Table Of Contents, right-click the layer and click Properties.
In the Layer ...
This is not possible with ArcPy. To access this property, you need to use ArcObjects. You can do this from python via comtypes.
The following example is from mbabinski1988 on Geonet
First set up comtypes and snippets.py:
Accessing ArcObjects from Python?
ArcObjects + comtypes at 10.1 and newer
Then the following function can be used to return ...