Tools that use VB expressions, such as Calculate Field, cannot use VB expressions in 64-bit ArcGIS products (ArcGIS Server, ArcGIS for 64-bit Background Processing, and ArcGIS Runtime).
Solution
- If your tool is not using a VB expression, you can ignore this warning.
 -           If your tool uses a VB expression, you must modify your task to  use  Python. Geoprocessing services and packages built for ArcGIS Runtime can use the following instructions to update the task:
- Remove the task from the Service Editor.
 - Edit the original tool.
 - Replace the VB expression with an equivalent Python expression.
 - Rerun the tool.
 - Add the result back into the Service Editor and finish publishing your service.
 
 
Tools executed in 64-bit Background Processing need to be rerun using a Python expression.