Add Assignment > Field Validation Rule: Need to select specific Assignment DFF on Target Field

Organization Name (Required - If you are an Oracle Partner, please provide the organization you are logging the idea on behalf of):

Marriott International

Description (Required):

We have a requirement to trigger an error message for specific field (Assignment Attribute 21)
However, as of 26A, we are unable to see the list of Assignment DFFs. Without this Target Field, the error message is triggering in When and Why section.

Here's a screenshot on Add Assignment > Validation Rules:

Use Case and Business Need (Required):

We need to trigger an error message for Assignment Attribute 21 (Assignment DFF).
If Action Date / Effective Start Date is less than today, and Assignment Attribute 21 is NULL, trigger an error message.