Getting error code of Add-WindowsCapability failed. Error code = 0x800f0954
Run “gpedit.msc” to edit your local computer policy.

Computer Configuration\Administrative Templates\System\Specify settings for optional component installation and component repair

My local policy seems to have defaulted to “Disabled” – after changing it to “Enabled” and selecting the checkbox labeled “Download repair content and optional features directly from Windows Update instead of Windows Server Update Services (WSUS)” the RSAT tools installed for me.

https://social.technet.microsoft.com/Forums/en-US/42bfdd6e-f191-4813-9142-5c86a2797c53/windows-10-1809-rsat-toolset-error-code-of-0x800f0954?forum=win10itprogeneral