Translation:The time to lower your guard has not arrived.
Apparently "abbassare la guardia" is an actual expression, but doesn't really make sense in this context, according to my italian gf.
The past participle of arrivare takes essere instead of avere. It doesn't really mean "is" here the way you are thinking of it.
It helps to think that all verbs that have to do with movement take essere, and the rest take avere to form the perfect. Similarly, the one that take essere also need to be conjugated to the subject in gender and number.