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
The "Allow users to create new records" toggle can be turned on/off for linked record fields or portal tables in the Create & Expand Records section.
2. The linked records field in Airtable itself does not allow linking multiple records.
Allow linking to multiple records might be turned off for the linked record field in Airtable.
On the form, the button will be greyed out in two scenarios:
It is not allowed to have multiple linked records
The maximum number of linked records has been reached
Both of these settings can be controlled in the Advanced section of the linked record field's settings.