Chosen Solution
I have this old iMac - it can run up to El Capitan and is currently running Snow Leopard 10.6.8. From the appstore I can now only get Sierra. Installing with an El Capitan DMG I get the error “This copy of the Install OS X El Capitan application can’t be verified. It may have been corrupted or tampered with during downloading.” The DMG has been verified and is all good. Any ideas please?
Try back dating your machine to just after when you down loaded. Go into System Preferences > Date and Time > turn off the automatic time set and move the date back. Do your installation then reset the clock.
