Is there anything wrong with linking two identical primary keys in two different tables?
It looks like your model is a one-to-one relationship between OrderNumber and OrderInfo, with OrderInfo.OrderID as the OrderInfo primary key and as a foreign key referencing OrderNumber.OrderID.

The error messages suggests you have defined OrderInfo.OrderID as an IDENTITY column. It should not be IDENTITY - the assigned OrderID value from thre related OrderNumber row should be used when inserting the related OrderInfo row to maintain the foreign key relationship. That should allow you to specify DELETE CASCADE.