Twill on the Rigid Heddle Loom
Keeping in mind that anything holding warp under tension is a loom, and any weave structure can be created on any loom, yes, you can weave twill with your rigid heddle loom.
Rather than "reinvent the wheel" I won't post a tutorial, but will link you to the Duck Duck Go search engine results. There are a lot of posts by other weavers showing you how. Enjoy!