I see a few issues with what you have here. First the oning actor which i guess is the character would not inherit feom point of interest so the cast would always fail. Second your array has one item which is index 0 but your getting the length of the array which is one, this will cause problems and not return the expected values.
To solve these issues i would create a variable of the pointofinterest type somewhere such as on the character then in the task you would just need to drag off the owning actor and cast to its class, then you could get the array directly feom there. Then in your macro you want to jave length minus one then plug that into the random int.
To populate your array you could use a get all of class node, or make the variable public and set the values in the array feom the scene details panel
Apologies for spelling here i wrote this on my phone