I had a post build command that worked just fine before I did an update on VS 2017. It turned out that the SDK tools updated and were under a new path so it couldn't find the tool I was using to sign my assemblies.
This changed from this....
call "%VS140COMNTOOLS%vsvars32"
"C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6 Tools\x64\sn.exe" -Ra "$(TargetPath)" "$(ProjectDir)Key.snk"
To This...
"C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools\x64\sn.exe" -Ra "$(TargetPath)" "$(ProjectDir)Key.snk"
Very subtle but breaking change, so check your paths after an update if you see this error.