After installing the March 5, 2018 update for Windows 10, KB4090913 (OS Build 16299.251), you may experience an error message when attempting to launch or install Windows Mixed Reality.
With Windows Mixed Reality already installed, when you attempt to launch the Mixed Reality Portal you may receive the error, "Something went wrong."
When installing or reinstalling Windows Mixed Reality you may receive an error, “We couldn’t download Windows Mixed Reality."
This issue impacts a limited number of devices after installing the March 5, 2018 update for Windows 10, KB4090913 (OS Build 16299.251). Microsoft has removed the update that has caused this issue.
A fix for this issue was included in cumulative update KB4088776. To resolve your issue:
Select the Start button, and then go to Settings > Update & security > Windows Update, and select Check for updates. Install KB4088776. Alternatively, you can manually install the cumulative update at http://www.catalog.update.microsoft.com/Search.aspx?q=KB4088776.
Launch the Mixed Reality Portal to complete setup.