We need to know exactly what question was asked. It could be five times the sum of y and 3. If that is the case, then the problem is saying that the very last thing that should be done is multiplication. According to the order of operations (PEMDAS), in order to write it correctly, we would have to put the sum in parentheses so it would be done first. In this case, we would write 5(y + 3). If you use the distributive property to simplify this, we would end up with 5y + 15 which is very different from 5y + 3.
Not sure if this was the original problem or not.
As far as i can tell there is no error.
I think the question is this:
A student writes 5y . 3 to model the relationship the sum of 5y and 3. Explain the error.
The word "sum" indicates that addition should be used and not multiplication. The student has used the multiplication sign instead of the +.
Hope this helps. :)