Hi
There can be a few issues with posting a sales order,
There are a lot more business rules and things need to be done in the correct sequence, personally i would suggest you rather created the order using a codeunit as that will allow you to control the creation better
If you have to use the page,
Go into NAV and manually capture a sales order, make note of the fields that you fill in then ensure that you write to the same fields from the web service
I cannot remember how we sorted it out but we also had a problem where the web servce was adding the fields in the wrong order so the item number before the type, this also causes errors
Using a codeunit is a lot simpler, create one for add sales header, returning the new SO number, then another adding lines, have your code create the header, then pass the lines with the SO number that was returned in the first bit of code
Cheers
Nev