AttributeError: 'DataFrame' object has no attribute 'Join'

Im new with AWS Glue. Im trying to left join 2 df then moving them to the S3 bucket. But I have a problem with left join syntax. Can you help me to resovle this?. Thanks all!

# Script generated for node Join
DropNullFields = DropNullFields.toDF()
user = user.toDF()
user = user.select("*", lower(col('OPRID'))).show()
left_join =DynamicFrame.fromDF( 
    DropNullFields.Join(
        user, 
        DropNullFields["erp"] == user["fin_erp_id"],
        "left"
        ),
    glueContext,
    "left_join",
)