Installer hangs on "waiting for other installations to finish" despite no other installations in progress
sudo rm /private/var/db/mds/system/mds.install.lock- reboot or
sudo killall -1 installd
Chances are that a previous installation hung and the lock was never cleaned up.
When an installation happens a daemon installd runs. Killing a leftover installd should allow another installation to happen (and will kill any new ones that are waiting for the defunct one to finish). So,
sudo killall -1 installd
Nice aspect of this is you won't need to reboot.
I had the same problem for no apparent reason and found a couple of solutions suggesting deleting some files - but nothing helped (at least was not enough). Eventually force-quitting the installer with activity monitor and then restarting my Mac did it and after the restart the installation completed fine.
Simply just check that no Applications are downloading in the App Store because I had iMovie downloading in the background and I didn't notice until later. After that I just cancelled the iMovie installation and the other installation downloaded within seconds.