Custom fields only letters are allowed

One more question is how to fix “Universal IDs Are Missing” – https://prnt.sc/_p29g5b_ypqM

You need to add a brand, MPN or GTIN to your feed. You can do so on the field mapping section of our plugin by mapping attributes to those fields that hold your product values for brand, MPN and GTIN.

I use brand attributes, but not all products have a brand – https://prnt.sc/VaytoQF97X0M

In that case you could create a rule that says something like this (use your brand attribute for it): https://www.dropbox.com/s/leiu0hhkvmu59nr/Screenshot%202022-12-20%20at%2015.04.12.png?dl=0