There was nothing wrong with your formula. In 30 years of using Excel, I've never encountered the @ as something required in any Excel formula, unless it was part of something being tested. Mind you, I generally prefer to an INDEX/MATCH combination instead of VLOOKUP because INDEX/MATCH works with unsorted data.
AFAIK, the @ is a compatibility option left over from the days of Lotus 123, which did require it. Excel allows the @ but does not require it and it adds nothing to what you already had, except clutter. If the Lotus compatibility character is what it is and this is supposed to have been an
Office course, your instructor was wrong.
If your instructor is going to tell you you're wrong, challenge him to produce an example where it does make a difference, and a reference to it. Simply telling you to "learn what it does" without providing any relevant material is unreasonable.
Even Microsoft's documentation makes no mention of it. See:
http://office.microsoft.com/en-au/ex...010343011.aspx
https://support.office.com/en-US/Art...rs=en-US&ad=US
http://msdn.microsoft.com/en-us/libr...ffice.12).aspx
FWIW, IBM - the owner of Lotus 123 - doesn't even sell or support it any more. The last release of Lotus 123 was version 5 ... in 1994!