This is because StyleDictionary is a constructor, not an object with a member function create. Use new StyleDictionary.create(/* arguments */).

Please see the documentation: getting started. Here, you will also find a couple examples with new StyleDictionary.create(/* arguments */).

This is the source code where StyleDictionary is defined: https://github.com/style-dictionary/style-dictionary/blob/main/lib/StyleDictionary.js.