Try opening terminal and running open $TMPDIR../C/
then delete the folder named com.apple.appstore while the app store window is completely closed. You may need to run rm -rf $TMPDIR../C/com.apple.appstore
if you cannot right-click + delete the folder normally, but I DO NOT recommend running the rm command unless you know what you are doing. After the com.apple.appstore folder is gone, reboot the computer and it should be able to install normally.
This solved my issue when I had trouble downloading multiple apps on a poor connection. I'd paused the downloads and tried to restart them elsewhere. That was a bad decision.
Hope this solves your issue.