Good day! I have been using Litecoin core since 2017. I decided to learn how to sign messages in Litecoincore. but I ran into difficulties and now I ask for advice. I am using Litecoincore v0.18.1. I enter the address, enter a test message and see the response in the form “The entered address is not associated with the key. You need to check and try again”. that is, the wallet says “you have no private keys”
I do not despair 
through the console I find the private key from the address I need, I try again but the answer is the same. the question is, is there a way to export keys from Litecoincore so that you have access to message signatures? (I take Electrum-LTC as an example, where after exporting the keys, the message can be signed).
P.S. making a backup copy of the wallet