HOW TO FIX LOCKDOWN.CPP:57 ERROR IN CYDIA IMPACTOR:
METHOD 1 – TRUST YOUR COMPUTER
This is the main reason why Cydia Impactor throws this error in the first place. When you connect your iOS device to your computer, you must “trust” it.
When you get this alert, simply tap the “Trust” button.
METHOD 2 – LOG INTO ITUNES
If the above fix doesn’t work for you, this will most likely fix it. Open iTunes on your computer, go to the Account option and Sign In with your Apple ID and password.
METHOD 3 – USE ORIGINAL CABLE
While not using the original cable is not much of an issue, it can make Cydia Impactor throw errors. Always use original cable and other accessories to minimize problems.
METHOD 4 – USE ANOTHER OPERATING SYSTEM
At the moment, this tool is incompatible with the 64-bit versions of Ubuntu. Therefore, you must use a different operating system to get it to work