I'm trying to run the code below via a script in a SQL agent job on a drive which is failing.  When i logon as the service account user and run it in an ISE shell it works fine which leads me to believe it's not access related.

I tried running it as a powershell job step but it wouldn't work so decided to run it as a cmdexec job type and call it like this: "C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe" -file "F:\Powershell\ScriptOutSSRSEncryptionKeys.ps1"

$ComputerName = "servername"
$KeyFolder = "\\servername\sharename\SSRSKEYS\"
$KeyPassword = "Password1"
$TimeStamp = Get-Date -Format "-yyyyMMdd-HHmmss"
Get-WmiObject -Namespace "Root\Microsoft\SqlServer\ReportServer" -Class "__Namespace" -ComputerName $ComputerName |
    Select-Object -ExpandProperty Name |
    % {
        $NameSpaceRS = $_
        $InstanceName = $NameSpaceRS.SubString(3)
        $KeyFileName = Join-Path -Path $KeyFolder -ChildPath ($InstanceName + $Timestamp + ".snk")
        $SQLVersion = (Get-WmiObject -Namespace "Root\Microsoft\SqlServer\ReportServer\$($NameSpaceRS)" -Class "__Namespace" -ComputerName $ComputerName).Name
        $SSRSClass = Get-WmiObject -Namespace "Root\Microsoft\SqlServer\ReportServer\$($NameSpaceRS)\$($SQLVersion)\Admin" -Query "SELECT * FROM MSReportServer_ConfigurationSetting WHERE InstanceName='$($InstanceName)'" -ComputerName $ComputerName
        $Key = $SSRSClass.BackupEncryptionKey($KeyPassword)
        If ($Key.HRESULT -ne 0) {
            $Key.ExtendedErrors -join "`r`n" | Write-Error
        } Else {
            $Stream = [System.IO.File]::Create($KeyFileName, $Key.KeyFile.Length)
            $Stream.Write($Key.KeyFile, 0, $Key.KeyFile.Length)
            $Stream.Close()
        }
    }

Error I am getting:

Executed as user: domain\svc_account. Exception calling "Create" with "2" argument(s): "Access to the path   '\\servername\sharename\SSRSKEYS\MSSQLSERVER-20150824-125254.snk' is denied."  At F:\Powershell\ScriptOutSSRSEncryptionKeys.ps1:24 char:13  +             $Stream = [System.IO.File]::Create($KeyFileName,   $Key.KeyFile.Length ...  +               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~      + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException      + FullyQualifiedErrorId : UnauthorizedAccessException     You cannot call a method on a null-valued expression.  At F:\Powershell\ScriptOutSSRSEncryptionKeys.ps1:25 char:13  +             $Stream.Write($Key.KeyFile, 0, $Key.KeyFile.Length)  +             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~      + CategoryInfo          : InvalidOperation: (:) [], RuntimeException      + FullyQualifiedErrorId : InvokeMethodOnNull     You cannot call a method on a null-valued expression.  At F:\Powershell\ScriptOutSSRSEncryptionKeys.ps1:26 char:13  +             $Stream.Close()  +             ~~~~~~~~~~~~~~~      + CategoryInfo          : InvalidOperation: (:) [], RuntimeException      + FullyQualifiedErrorId : InvokeMethodOnNullException calling "Create" with "2" argument(s): "Access to the path   '\\servername\share\SSRSKEYS\RSDYNAMICS-20150824-125254.snk' is denied."  At F:\Powershell\ScriptOutSSRSEncryptionKeys.ps1:24 char:13  +             $Stream = [System.IO.File]::Create($KeyFileName,   $Key.KeyFile.Length ...  +               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~      + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException      + FullyQualifiedErrorId : UnauthorizedAccessException     You cannot call a method on a null-valued expression.  At F:\Powershell\ScriptOutSSRSEncryptionKeys.ps1:25 char:13  +             $Stream.Write($Key.KeyFile, 0, $Key.KeyFile.Length)  +             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~      + CategoryInfo          : InvalidOperation: (:) [], RuntimeException      + FullyQualifiedErrorId : InvokeMethodOnNull     You cannot call a method on a null-valued expression.  At F:\Powershell\ScriptOutSSRSEncryptionKeys.ps1:26 char:13  +             $Stream.Close()  +             ~~~~~~~~~~~~~~~      + CategoryInfo          : InvalidOperation: (:) [], RuntimeException      + FullyQualifiedErrorId : InvokeMethodOnNull.  Process Exit Code 0.  The step succeeded.

You cannot use a remote share in a remote session.  It is not allowed for security reasons.

I have lots of jobs that copy files from shares to other shares how can it be different for this one?

If you are not trying to run it remotely then the account it is running under does not have access to that path.

Check you permissions.