There are 4 possible reasons why the Create New button is not showing up for the linked records field or in a portal table:
1. Creating a new record is turned off
2. It is not allowed to have multiple linked records. This is only applicable in the form.
3. The maximum number of linked records has been reached. This is only applicable in the form.
4. The linked records field in the Airtable itself does not allow linking multiple records.