Is there actually an SD slot with a card in it? Usually part of the internal storage is called "/sdcard" (for historical reasons).
If it is the "internal sd card" then modern android versions usually allow you to use all of the internal space for apps or media, but the actual SD card (if there is one) only for media. However, there are a few (usually off-brand budget devices) where for some reason the manufacturer is still using a Gingerbread-era storage model where they allocate a small part of the storage for apps and app data and leave the "internal sd card" for media only. Can you give us a full description of what your tablet says about your storage?
To access this space, you could use a file explorer app, or see whether your apps allow you to set the location for their storage. Or copy media to it directly from a computer. It might be that your tablet lets you move part of your apps there (though most do not these days) - go to Settings > Apps > Downloaded, select one of your apps and see whether there is a "move to sd" button, and if so whether it works or is greyed out (if it exists but is greyed out try a few others, since not all apps can be moved even if the option exists).