LEN argument must be a variable or string expression
You called the Len function and specified as its argument something other than a string expression or the name of a variable. For example:
Print Len(123) ' Illegal because 123 is a numeric constant
Print Len("123") ' Legal. Returns the number of characters in
' the string "123" (3).
Dim X As Integer
Print Len(X%) ' Legal. Returns the number of bytes allocated
' to store an integer value in memory (2).
Make the argument a string expression or the name of a variable.