If you see an error saying you are running game version 1.6.1170 but need 1.6.640, Steam likely auto-updated your game. You will either need to download the latest SKSE version (3.0.x+) or use the Skyrim Special Edition Downgrade Patcher on Nexus Mods to go back to 1.6.640.
While Bethesda has since updated the game to 1.6.1130 and 1.6.1170, the 1.6.640 build remains incredibly popular because of its stability and the massive library of compatible mods. skse 16640 link
By staying on 1.6.640 with the corresponding SKSE link, you ensure: If you see an error saying you are running game version 1
Many modders intentionally "backport" or freeze their game at 1.6.640. This is because the transition from 1.6.640 to 1.6.1130 involved significant changes to the way the game handles plugins (the "Marketplace" update). By staying on 1
Here is everything you need to find the right link, install it correctly, and troubleshoot common issues. The Official SKSE 1.6.640 Link