Windows Server 2012 R2 - Single Server Session Based deployment

I have a single server running on Windows Server 2012 R2. I have no problem creating a new session based deployment to this single server via the Server Manager UI - however, when I try to automate the process in a Powershell script it doesn't work.

Here is my script:

import-module RemoteDesktop
New-SessionDeployment -ConnectionBroker "$env:computername.$env:userdnsdomain" -WebAccessServer "$env:computername.$env:userdnsdomain" -SessionHost "$env:computername.$env:userdnsdomain"

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It yields the error:

New-SessionDeployment : Validation failed for the "RD Session Host" parameter.
MY.OMMITTED.FQDN      You cannot restart the local server.
At line:1 char:1
+ New-SessionDeployment -ConnectionBroker "$env:computername.$env:userdnsdomain" - ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,New-SessionDeployment

I tried all the proposed solutions for this problem available online, including running the script as Administrator, setting my execution policy to unrestricted, remoting into the local server via

Enter-PSSession $env:computername

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and then running the script. But all of these proposed solutions yield the same error message.

All I'm trying to do is automate what I can already do via the Server Manager UI.