What transaction type did you use. The transaction type in your batch does not only function as a kind of number group for your financial document, but it also indicates what kind of transactions you can execute.
If you want to enter purchase invoices, then you need to use a transaction type with transaction type category "Purchase Invoices" or "Purchase Credit note". Also on the transaction type you can enter the first and second session the system should start. When creating a transaction type of category 'Purchase Invoices', the system will default the correct first session (tfacp110s000 (or tfacp2600m000 from FP8 onwards)).
I suppose that you have used a transaction type of category 'Journal Voucher' and with such a transaction type you can ony enter plain journals.
Please read the helptext on transaction types.