Issue In billing

Hi Sap Gurus,

Could any one help me out in this issue on billing

I have created a sales order with with item category TAB (Individual Purchase Order). i have put order related billing . When ever i reach billing an error is coming this item category TAB cannot be invoiced with billing type F2. Could any one help me out with which billing type i have to use TAB.

Thanks in Advance

Edie