@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!

Please tell me how to unblock it, because the function is given only to change the password in Litecoin Core.