Illustrator 2020 can`t change preferences from pt to mm

The Preferences unit setting only works for new documents that do not have a unit, like when you open an image or a pdf without Illustrator editing capabilities.

The document units can be set when creating a new document or will be set when a new document is based on a new document preset with the desired unit.

They can be changed for open documents in the Preferences or document setup.

But the Preferences setting will automatically change to the unit of a new or existing document.

So the best thing to do is create a New Document Preset with the desired units.

You want new documents with mm? You have to change that in the New document profiles. https://helpx.adobe.com/illustrator/kb/create-custom-new-document-profile.html