I suspect you didn’t quite follow the adding a virtual map section correctly. It has to be “hash:/etc/postfix/virtual”. The “hash:” part has to be in there. This is the error you’ll get if you either didn’t include the “hash:” or you didn’t do the step at all.
For some reason this is a very common question. Folks just cannot believe the "hash:" has to be there.
And, just to be sure you’re looking at the docs I’m talking about here it is: