It looks like your script can't access the file, or it doesn't exist. Check to make sure C:\Octopus\Applications\CSIS_DEV\ESG.CSIS.StartUp\38.2.19.0\ESG.CSIS.StartUp.exe
exists and is accessible to the user the tentacle runs under (use windows effective permissions).
Also make sure it's there by the time the script runs, you haven't mentioned whether this is a seperate step, PreDeploy.ps1, Deploy.ps1 or PostDeploy.ps1 script