First have to create Journal
Go to "Accounting" menu in that click on "Journal" sub menu.
Enter the required fields. Note:The Default Credit/Debit Account must be same.
If Default Credit/Debit Account is not display, Go to Setting->Users & Companies->Enable "Show Full Accounting Features".
Because of "Invoice" not be in the state of Paid.After that Create POS.
Note: check "Is Pay Later" checkbox to enable the "Pay Now" button in POS. If is not checked you cannot make a payment.
Be sure that your accounting settings are given as per the below image, because if the settings are not given it will be proceed like a normal process.