I tested your setup and it works fine. The fact that your $gv(json)$ is empty (on your 2nd screenshot) means the Flow has never finished successfully.
Open your Flow and tap ‘Test flow’ (the Play icon).
Look at the log. Does it say ‘Done’ or does it show an ‘Error’?
TIPS
Check the URL address typed in the Flow action, a URL address should not have space characters.
I hope this would help.
