Don't know why @Janos deleted his answer, but it's correct: your data frame Train
doesn't have a column named pre
. When you pass a formula and a data frame to a model-fitting function, the names in the formula have to refer to columns in the data frame. Your Train
has columns called residual.sugar
, total.sulfur
, alcohol
and quality
. You need to change either your formula or your data frame so they're consistent with each other.
And just to clarify: Pre
is an object containing a formula. That formula contains a reference to the variable pre
. It's the latter that has to be consistent with the data frame.