When referencing only EPiServer.Forms.Core package in your project, results in EPiServer Admin user interface fail to load.

Browser console output - JS fails to load following client resources:

http://localhost:4225/EPiServer/Shell/9.9.1.0/ClientResources/epi-forms/component/FormsContentTypes.js

http://localhost:4225/EPiServer/Shell/9.9.1.0/ClientResources/epi-forms/component/Forms.js

As result - you will see only blank screen with top main menu.

Adding rest of EPiServer.Forms.* packages - restores Admin UI.