Hi,
You should have folllowing settings (For example) for WRX key in OBYC t.code for your chart of account:
Valuation Class-------Debit---------------Credit
BLANK-------------------G/L account-----G/L account
3000---------------------- G/L account----- G/L account
Also keep following Posting keys
86-GR/IR debit
96-GR/IR credit
Regards,
Biju K