If you have changed the eDocBuilder template to which the product in your Saved for Later area was assigned to, then it will display an error when trying to Customize it. Once you have changed the template assigned to the product, it causes a mismatch in the session because it's still referring to the old template.
There are three ways to resolve this:
1.) Remove the item from your Saved for Later, then add it again.
2.) Re-assign the original template back to the product
3.) Start with a clean session.
When you Customize a product in Pressero, you create a "session" in eDocBuilder for the particular order you're working on. This session creates a "snapshot" of the eDocBuilder template as is throughout the order process. Changes made to an eDocBuilder template after a session is created are not all inherited into the open session. As a best-practice, to use the changes made to an eDocBuilder template, you would need to start a new session after the changes have been made.