You have set up a Processing Constraint prohibiting updates to an extensible flexfield in a...

A processing constraint is a rule that controls the changes that can be made to an order or a fulfillment line at various stages of the order-to-cash process.A processing constraint can be based on various attributes, such as the order type, the fulfillment line status, the ship-from warehouse, and the extensible flexfield1.An extensible flexfield is a custom attribute that you can use to capture additional information that is not provided by the predefined attributes2. To set up a processing constraint based on an extensible flexfield, you need to run the “Publish extensible flexfield” process after you create or modify the extensible flexfield.This process enables the extensible flexfield to be visible and usable in the “Manage Processing Constraints” page3. If you do not run this process, the processing constraint will not be enforced when a user updates a fulfillment line that meets the conditions.

References:

Overview of Processing Constraints

Overview of Extensible Flexfields

Create Processing Constraints Based on Extensible Flexfields