In unit price column when I am entering 150 its working perfectly, but when I am changing it to 150.50, an error message is coming " Vat Posting Setup does not exists"
Dear Prakash, Could you be more specific please? What version of NAV are you using? Does it happen and when your unit price is either 151 or 149? Does it happen to invoices for all of your customers or just one? Do you have any additional code besides NAV standard code to your form or relevant table?
How is VAT setup in the GL, and do you adjust VAT for payment discount? Sounds like a rounding issue triggering the message but it could easily be a limit as Daniel suggests, or a localisation to the VAT.