El Capitan: Appstore says "downloaded" and "there's been an error" (stuck)

Solution 1:

Apparently it downloaded the OS X El Capitan installation (under that name, or Install OS X El Capitan) in the Applications folder, which seems to be the download location. I could launch it from there.

Problem solved…

UPDATE: if this would occur during you Mac OSX Sierra download, the file you'd be looking for is "(Install) macOS Sierra" in the Applications folder.

Solution 2:

me too. On my 2009 iMac, found it under Install OS X El Capitan in the Applications folder. Wasn't sure what was happening as it seemed to be stuck on Downloaded on the Mac App Store, and re-starting my computer wasn't doing anything. Seems to be running now