I resolved this problem with the following procedures. I don't know if this was the correct method, but it worked in this instance.
1. Go to Help >about this page
Under Page information the "Page:" row will tell you what database table the page is based on.
Under "Table Fields" look for the No. Series row. This will tell you if there is a number series already designated for this Sales Order page. My row was blank.
2. If you have access to the development environment, then you can open the object designer and the database for NAV.
3. Find the table the page is based upon. For me it was table 36 Sales Header. Right click and choose run, this will open another instance of NAV and bring up the table.
4. Search for the record you are having trouble with.
Look in that record for the column "No. Series".
5. Change the No. Series field to the correct No. Series.
6. Close out the new instance of the table, the second instance of NAV and the development environment.
7. Then go back to the sales order and it should post.
If there is another way that would be a preferred method, I would love to know. I still have not found out why this occurred.
Good Luck!