The most common way to acquire the 1.5.97 tools is through the repackaged version available on the Nexus Mods website or via the method.

When you download the " Skyrim Special Edition: Creation Kit " from Steam today, you are actually downloading a version built for the latest Anniversary Edition (1.6.x+). This new CK saves plugins in a format (version 1.71) that is not backwards compatible with version 1.5.97, leading to "Unable to find masterfile Skyrim.esm" errors or immediate crashes upon launching the game. How to Get the 1.5.97 Patched Creation Kit

To use the with Skyrim Special Edition version 1.5.97

Using a patched Creation Kit with Skyrim Special Edition version 1.5.97