This message shows up when you try to sign an invoice and have checked the Sign check-box for account posting lines that you are not authorized to sign. It can be because you don't have permission to the account posting, or because you don't have a sufficient authorization amount. 

The system checks the boxes for the lines that you have permission to sign, you never have to check the boxes yourself. If the lines haven't been signed and your role is the last in the flow, you will need to add another role to the flow in order to sign the invoice.