For those who are still having the issue of “add PRODUCT ID column, here’s how I fixed it.
I’m switching my art shop from 7.1 to the fluid engine. 1. export the file from squarespace "inventory" (the original site) 2. "import" the file into Excel.
3. make all changes without removing anything except all the listing under PRODUCT ID & VARIANT ID - make sure everything under those 2 column are blank. 4. Save as (comma separated values) .csv not the .CSV UTF-8 (comma delimited) because it won’t work. 5. Before you import the file to the site, add a listing (this something you’ll delete later) - make a fake listing with the basic information.
6. Then "import" the file to the squarespace inventory. PS: I did not change the name of the file saved.Hope this helps others.