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. It is not allowed to have multiple linked records.
This is only applicable in the form. In the Advanced section of the linked record field's settings, you can choose to only allow linking of a single record.
3. The maximum number of linked records has been reached.
This is only applicable in the form. In the Advanced section of the linked record field's settings, you can limit the maximum number of records to link in the field.
4. 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.