Translation:It is not the time to lower your guard.
With no qualifying pronoun, how are we supposed to know that "la guardia" should be "your guard"?
"let down your guard" is standard idiomatic English (with 68,800,000 Google results) and should be allowed.
Because a pronoun other than 'your' has to have a back reference as "It is not the time for John to lower his guard".
I'm not sure this is the case. I think it could just as easily be "our guard". Also, Duo routinely delivers sentences with "suo" that lack an antecedent, so I don't think this rule can help users know how to answer the question.
I think you are right. Quite possibly without context there is no determinate sense - 'our/youjr/his/my guard. Is Non è il momento di abbassare la mia guardia ( .... to lower my guard) correct. I find that you can get stuck on things like this in Duo!