"The juice is bad."
Translation:Der Saft ist schlecht.
In German when we say something is "bad" we usually mean that it is in some way defective of it's original design. So for example a "bad" television is a broken one. However, if the same television were being described as "not good" having the context of being "good" means it still works but we should probably buy a new one. So for this example "bad" juice is juice you won't drink, "not good" juice is juice you would drink but not prefer.
"not good" doesn't have to mean "bad". I think it is better to give the literal translation "schlecht"
Actually, you might hear "Der Saft ist nicht gut" in many situations where people are trying to convey that the juice is bad. Often, the indirect, reserved/polite statement may be preferred to a plain "Der Saft ist schlecht!" However, this is a cultural, not a linguistic issue, which DL probably cannot make individual allowance of in a language course.