Hello ,
Thank you for posting in the community and sorry to hear about the troubles. There are some materials with transparent settings that cannot be selected in the view port but if you select them in the scene graph or use the material picker it should allow you to edit the material. You should also be able to find the material in the galley view of materials.
The drag and drop function should work on all materials thus if you are still having issues maybe there is a bug and we would need to investigate this matter further. Would you then be able to use the [ contact us ] button from the home page or follow this link: https://twinmotionhelp.epicgames.com/s/case-community-page
Kind regards,
Vincent B.