If you already have an Ecwid account with existing products, you can still connect it to Printful. However, your products will only show up in Printful if they have combinations (variants). See how to set up product combinations (variants) for Ecwid in this video tutorial.
All products that are created with Printful's product push generator will automatically have product combinations (variants) if you’re a Business or Unlimited plan user.
Ecwid Venture plan users won't be able to add all products automatically with the push generator. Read more here: Do some Ecwid plans have limitations?