The adjective indeterminable describes something that can't be known for sure, like what grade you would’ve gotten on a paper you never turned in.
To determine is to calculate something, make a final decision, or to be sure. Something that's indeterminable is none of these — it’s not able to be determined. Maybe you need more information or better math skills. The grade you would’ve gotten on a paper you didn’t write is indeterminable, but if you’re lucky your teacher will give you an extension. Then that shining “C” is all yours!