Explanation
Correct code block:
transactionsDf.sample(withReplacement=False, fraction=0.15).select(avg('predError')) You should remember that getting a random subset of rows means sampling. This, in turn should point you to the DataFrame.sample() method. Once you know this, you can look up the correct order of arguments in the documentation (link below).
Lastly, you have to decide whether to use filter, where or select. where is just an alias for filter(). filter() is not the correct method to use here, since it would only allow you to filter rows based on some condition. However, the question asks to return only the average prediction error. You can control the columns that a query returns with the select() method - so this is the correct method to use here.
More info: pyspark.sql.DataFrame.sample - PySpark 3.1.2 documentation
Static notebook | Dynamic notebook: See test 2