Navigate to Administration>Registers>Accounts and search the account that you want to have as an distribution account.  

Click on Search, and then on the row that you get up.
This will take you to the below image, and here the box for "account can be used for distribution account" should be checked.
Please note that this information usually comes from your ERP system and should be handled as possible from there.
If you can not handle it from your ERP system and you manually check the box in Palette, then it may happen that it is overwritten, and the tick is removed on the next integration operation. Register a ticket in Palette Helpdesk if this happens.