Which type of Discovery license do you have? If you have floating license, then Discovery will automatically pull Granta materials license. You need to ensure that the material option is correctly set from the Options panel of Discovery. For that, go to File menu - Settings panel. In Physics tab, go to Materials section and select Library option as "License and Included". See below snapshot. The "License" option fetches the Granta materials.
If you have a Named user (subscription) license if Discovery, then you will not be able to fetch Granta license and its materials.