Try adding the following index when running my query:

create index [ix_SalesInvoice_SalesComparison]
on [dbo].[SalesInvoice] ([ShipDate])
include
(
  [BCustNo]
, [SalesAmount]
, [AVGCst]

Why are you posting new threads when you have already asked this question and been given an answer in another post.  You aren't helping yourself doing this.

Here is...

The following is what I came up with based on your original post.  Please give it a try without adding the NOLOCK hint.

declare @StartDate date =...

It is telling you exactly what is wrong in the error message.  There is an identity column in the tables and you are trying to insert an identity from one...

Just checking to see if you got the answer you were looking for her.

If it is the first row where PreviousTermDatePlusOne is NULL, that is reason; 20160701 <> NULL is not true therefore the result returned is 1.  Actually that statement isn't false...