How to create a list with checkboxes in Automator?
Not with Automator

This is not possible using only Automator and an Open Scripting Architecture (OSA) language. Automator is not intended to present arbitrary user interfaces.

Alternatives

An alternative approach would be to create an Objective-C/AppleScript application, or a local web page and JavaScript.

There appear to be third party tools, such as Dialog Maker, that may help.