I've never seen that exact error message (which version of matlab?) but your code is certainly going to produce an error if there is no match for an element of A since you'll try to assign empty to a scalar. And if there is more than one match and Data has more than one column, max(Data(I, :) is going to return a vector, which will also fail to be assigned to a scalar.
How to fix that depends on what you want to do in either case.
Also matlab tells me that "strmatch is not recommended. [...] To find an exact match for a string, use strcmp." Certainly, the way you're using it strcmp would actually be better.