Warning message on purchasing Quickbooks Version 6

Hello people,
I have just recently upgraded to QuickBooks pro 2006, and when I want to close a purchase order that I created with the old version of Quickbooks which is ver. 6, I get an error message that is telling me about "you must assign an account for this transaction",
Please refer to the screen shot below.
Warning
You must assign an account for this transaction.
When I analyzed the error message above, I am thinking about compatibility issues, but as I asked the vendor there no problem with compatibility, since it can open all files from previous versions,
Please help me solve this error that I encountered,
Thanks.
