Three reasons a field is not in Choose fields, in the order to check them.
It is on a different kind of row
Variants, products and orders each have their own Choose fields. A metafield defined on products is not listed under Variants. From Home, press the card of the kind of row the field belongs to.
It is of a type the app does not edit yet
Look under Fields that can't be edited here yet at the bottom of the list. A list, a reference, a date, a URL, a colour or JSON is shown there and cannot be ticked. See Which fields can be edited.
It has no definition
Choose fields lists metafield definitions. A metafield written by an app or by a theme without a definition does not appear. Make a definition for it in your Shopify admin, under Settings, Custom data, with the same namespace and key, and it is listed the next time you open Choose fields.
Orders
Orders list no Shopify-own fields beyond Note, Tags and PO number. An order's customer, address or line items are not edited here.