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.