Question:
How can I get the ACPF to work with ArcGIS Desktop 10.4? I am having issues with the DEM Fill process and was hoping the reason for the error was because I am using Desktop 10.4.1.

Answer:
This error may occur if the workspace and/or scratchWorkspace settings are pointing to bad pathnames. Check the workspace environment settings in ArcCatalog to make sure they are valid. If they are unassigned, you may want to create your own folder/file-geodatabase set and assign the settings there. Generally, the default workspace and scratchWorkspace work well, as long as they are not in a network location. Additionally, review the documentation regarding pathnames and other setup procedures. Ensure that you are not using network drives or have spaces in the pathnames to the toolbox or the database.