Incoming Payment msg

I am trying to enter an incoming payment in the OEC company. I selected the check box next to the invoice number and clicked the "Add" button. I click the "Add" button and keep getting an error msg..."...$ of the amount to be paid is not matched to existing transactions. Do you want to post this amount as a Payment on Account?". i don't understand why SAP is asking me if I want to create a payment on account when i've already marked the "selected" box next to the document number.