When I click the "+ Add New" button the option to use Create is greyed out and unavailable.
The Create option is only available when the required fields are filled out.
The availability of the "Create" can change depending on item and field customization. If you have filled out the required fields in the "+ Add New" ribbon and the "Create" option is still unavailable then that item type has required fields not displayed in the ribbon. In that case the "Create with Details" option must be used instead.