@zaratu You need to unlock your wallet first.

However, DON'T pre-export your key. It's fine to dumpprivkey on some new, zero-balance address that you just made to test.

For maximum safety, you don't want to even see that key until you've already moved the funds to a new address after the fork. This is just in case your PC is compromised and something can capture your screen or clipboard; ideally we don't even see that key until it's not holding LTC any more.

Practice safe forking!