This is not An Array but you are using it as One
From Xojo Documentation
Error message
You passed an object to a method or function that expects an array but did not get one. It also occurs if you are trying to assign a value to an element of an alleged array but it is not an array.
Examples
Dim aInts(4) as Integer
Dim f as FolderItem
Redim f(10) //not an array
Dim a as String
a(1)="Ted" //not an array
Dim f as FolderItem
Redim f(10) //not an array
Dim a as String
a(1)="Ted" //not an array