Mac App Store: Downloads just don't start (Mojave)
I believe there can be multiple reasons for this behavior and various solutions. Too bad that app store simply does not give any indication of what is wrong with it.
The problem happened to me too. Nothing from this topic helped me, including restarting the App Store, logging in and out of the App Store, doing the above after opening a new connection with a VPN, much closer to California (I'm in Europe).
The only thing that worked is going to Activity Monitor and force-killing appstoreagent process. The app started to download at once. After force-killing appstoreagent, there was no need to log out and log in again in the App Store. Updates and downloads just work again.