Please, follow our best practices when asking for help.
The error seems to be self-explanatory, you are trying to access a column named :y1 in a dataframe, but it does not have a column with such name, the column with the most similar name is :y. Should your dataframe have a column named :y1? It is impossible to me to say without a Minimal Working Example (MWE). Also, are you trying to get a vector or a single value from the dataframe? (the syntax [a; b] allows a and b be either elements or vectors).