value saveAsTextFile is not a member of org.apache.spark.sql.DataFrame
This is not standard part of the API of DataFrames. You can either map it to a RDD, join the row entries to a string and save that or the more flexible way is to use the DataBricks spark-csv package that can be found here.

If it's just one column you can map it to a RDD and just call .saveAsTextFile(filename)