Hi Imran,
The issue is NOT with the exchange rate maintenance in T. code OB08.
Please check first the data you are entering for purchasing.
The error message is coming because it is not able to identify the source currency ( / USD) and also the date for which the exchange rate need to be determined (that is why 00.00.0000 is coming).
Please correct YOUR INPUT DATA first.
This should resolve your issue.
BR
Amitash