In Settings I can choose Developer mode, but the Enable Device Portal and Device Discovery controls are greyed out. As soon as I enable Developer Mode by choosing the radio button in Settings on Window 10 Pro on a laptop, I get the following message:
"Developer mode is turned on. However remote deployment and Windows Device Portal couldn't be installed so they are not available. Error Code 0x800f0950

The Windows OS is Insider Preview, Build 17025_prerelease.170120-126

on a related issue I do not find webmanagement.exe