If you are using Avast antivirus software and are unable to launch Lineage II because Avast blocks the game due to GameGuard, the following steps will help you resolve the issue and let you run Lineage II successfully.
Step 1: Activate Silent/gaming mode
Right-click on the Avast icon and select "Silent/gaming mode".
Step 2: Add the Lineage II game folder to the Exclusions list
In the Avast user interface window, click "Settings".
In the Settings window, click "Tools".
Where is says Sandbox, click "Customize"
Click on Exclusions and browse to find the NCSOFT\Lineage II folder.
Click "OK" to set the exclusion.