Problem with Retrieve variants under a gnomAD allele frequency threshold

[JFL9421]

I was able to "Read Nirvana JSON output by lines" following the provided python instruction. However, when I tried to "Retrieve variants under a gnomAD allele frequency threshold", I got the following Error Message
"Traceback (most recent call last):
File freq_df_01.py", line 8, in
for position in positions:
NameError: name 'positions' is not defined"
What should I do to make it work? If it worked, how can I export an output json file containing only the data that are under the defined gnomAD allele frequency threshold? thanks in advance!