If you are trying to redeem your bitcoin cash and you come across this error message in Electron Cash, it means that you are redeeming from an address after 1 August 2017.
Check the address of the private key you are importing and back track to a transaction that dates prior to 1 August 2017.
Yeah … so this is a problem. Because … well … stuff happens after August 1, 2017. So what happens if you buy Bitcoin Cash and throw it onto a paper wallet and need to import it? It will be after the fork. Is there seriously no way to import a private key that didn’t already exist on August 1? Kinda seems a little shortsighted on everyone’s part no?