How to create a job that uses Powershell script?
You need to pass the powershell file in as an argument 

powershell.exe -file C:\Users\domainAccount\Desktop\deleteold.ps1 -FolderPath \\192.168.x.x\Backups -FileAge 2 -ListOnly -LogFile \\192.168.x.x\Backups\log.log


Or if the step type doesn’t have to be CmdExe you can change the type to PowerShell