Today I did an upgrade on ConfigMgr Current Branch from version 1602 to 1606. During the upgrade I choose to validate the client package in pre-production first. After the upgrade however, there was no possiblity to promote the ConfigMgr client to the latest version. I restarted the ConfigMgr server and console, but without any luck. After that I decided to install Update Rollup 1, which was successful again, but still the same behaviour. No possibility to promote the ConfigMgr client to the latest version again.
Lucky me I found the following thread: Microsoft TechNet
It mentioned: I solved my problem. It seems to be a RBAC problem. The user I used in ConfigMgr was "Full Administrator" but assigned though an AD group and not directly assigned. When I added my user directly as a user account in the ConfigMgr console and gave him "Full Administrator" rights then I could promote the client to production.
In my case (customer location) this was the same situation. The user which did the upgrade and was logged on is part of an security group. That's a nasty situation for sure! Long story short: I added the user directly as Full Administrator in the console, and the "Promote Pre-production Client" button became available again. Hope that Microsoft will fix this for future builds, because adding users instead of groups is not the way to go. Hope it helps!