Bethesda does not host old versions on their official launcher anymore. The Steam Launcher will always force the latest version (currently AE). Here is how you source the correct version.
Warning: Do not try to open a 1.6.640 (AE) plugin in this CK. You will break it. creation kit skyrim se 1.5.97
A massive portion of the modding community refused to update. They downgraded back to 1.5.97 using tools like the Unofficial Skyrim Special Edition Downgrade Patcher . Bethesda does not host old versions on their
Once you have the right executable, the vanilla CK will still crash constantly. You need these patches: Warning: Do not try to open a 1
: If you receive "Chunk size too big" errors, these can generally be ignored or suppressed using SSE Creation Kit Fixes Missing Scripts : If you cannot see source scripts, ensure the Data/Scripts/Source folder contains the files extracted during step 2.4.