Yeah, I had this same problem... Started off with Xcode then a few other applications after updating my MacBook to High Sierra.
There seem to be two options in order to fix this:
1) Remove the application to the trash (fastest method)
2) Just go back to the application on the Apple Store and click download again (can take a few tries)
I guess we will have to wait a little while for this to be a stable Operating System, but hay its a lot faster than any issues that need to be fixed on any Windows Operating System, not that I am being biased as I use both Operating Systems on a day-to-day basis 😝