primary_index is required argument for partitionedBy so use options to set primary index.
PySpark
pyspark_df.writeTo("testdf_writeTo").partitionedBy( days("datetime") ).create()
teradatamlspk
teraspark_df.writeTo("testdf_writeTo").options( primary_index=(teraspark_df.Feb, teraspark_df.Jan)).partitionedBy( days("datetime") ).create()