Why Can I Not Post an Invoice?

When using the Post 1 or Post All button I am receiving this prompt, why?

Solution

Because the terms of the account is set up as Cash Sale, it requires payment at the time of posting.  The prompt is asking you to open the invoice and receive the payment.

This is done by either selecting Process or the Payment button on the top left.

If the Customer is not paying at the time of posting, you will need to change the Payment Terms of the Account.