If a customer couldn't edit their order, it's usually one of these:
Window closed - the edit time limit passed, or the order was already fulfilled.
Display conditions - the order didn't meet your rules (e.g. a minimum cart value, or an excluded tag/SKU/subscription).
Feature off / test mode - Order Editing is disabled or still in Test Mode.
Blocks not added - the "Edit Order" block isn't on the Thank You / Order Status page.