Open your Google Sheet, go to the Add-ons menu and choose Document Studio > Open to open the sheet sidebar.
Inside the sidebar, go to the Google Forms section (after mail merge) and check the Enable option to generate PDFs from Google Forms. Here you can also choose Google Forms that should generate PDFs.
Once you have made the selection, go to the Finish and Merge section and choose Save to turn on the Form Submit trigger.
We have released an update and that should have restored the Merge on Form Submit option to its original location.
Do you have a link to this update or is this only with a paid license. I don't see it and the above solutions is not available either. my version is 17.4.0