Hi Sofía, thank you for your question!
The difference between Must Be TrueX and Cannot Be True questions stems from the nature of the correct and incorrect answers.
In a Must Be TrueX question, you will have 4 incorrect answers that Must Be True, with 1 correct answer that is Not Necessarily True.
In a Cannot Be True question, you will have 4 incorrect answers that Could Be True, with 1 correct answer that Cannot Be True.
As you can see, they're not both asking you to identify something that Cannot Be True, since Not Necessarily True is distinct from Cannot Be True.
This distinction is further explored in posts such as this one that discusses Logical Opposition (viewtopic.php?t=7011
), as well as this one that further explains your specific question (viewtopic.php?t=2772
I hope this helps!