How to substitute for mustard seed<br>Use prepared horseradish to add spice. Prepared horseradish is made by combining ground horseradish with vinegar, and sometimes other spices, to form a sauce. You can get it from your local supermarket.
Most supermarkets sell prepared horseradish.
It is correct as the context tells us you can get prepared horseradish at your local supermarket.  The AI probably missed it because it didn't infer that supermarkets are selling horseradish from the statement that "You can get it from your local supermarket".
( z1 sell :ARG0 ( z2 supermarket :quant ( z3 most ) ) :ARG1 ( z4 horseradish :ARG1-of ( z5 prepare ) ) )