I imagine you're referring to this test question:
Esos pendientes ___ entre 40 y 50 euros al menos. (Those earrings probably cost between 40 and 50 euros at least.)
There are two correct answers for this: deben costar and deben de costar
Both deber and deber de can be used when we are expressing a strong assumption as explained in the lesson. Both options from the menu need to be ticked in order to get a completely correct answer. If you only ticked one of them the system considers this as "nearly correct". Watch out for hints saying [one, some or all may be correct].
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Test your Spanish to the CEFR standard