: If Steam has auto-updated your game beyond 1.6.640, you may need a "downgrader" tool if you specifically need this version for mod compatibility. 2. Download the Correct SKSE Files

Using the wrong version will result in a "Version Mismatch" error as soon as you try to launch the game. You can always find the latest builds on the official SKSE website . Why version 1.6.640 matters