Hello Ahmed,
In a nutshell, you need to setup the Dynamics AX intercompany accounts in GL first before you can process intercompany trading scenarios such as intercompany purchase order and intercompany sales orders.
Please note that the intercompany accounts that you setup in GL are not only use for intercompany trade scenarios but also for other, non-trade related intercompany transactions such as e.g. intercompany allocations and other intercompany postings.
Hope it helps,
Ludwig