This is a bug in the Mac App Store. It's happened to me several times with different apps.
I've contacted developers about it and they told me the only resolution is to quit all apps and do a hard restart of the Mac (shut down then restart, don't just do a restart).
Doing that worked for me, although sometimes I have to do it twice to get it to work. I don't know the root cause, I only know that this workaround seems to do the trick.
If you search this site you'll find many similar issues. You can also try the answers already provided. I didn't mark your question as a duplicate because I'm not sure if the cause is the same as those.