Please make sure the account is set up for InterCompany as follows:
If using Ribbon style menu, click System, Options, Nominal Ledger Options.
If using Module/Classic style menu, from the navigation bar, click System Control, Nominal Ledger Options.
Select Plug-Ins, InterCompany, InterCompany Options
On the InterCo. tab, see whether the account is available here, if not then it will need adding.
To add a new entry, scroll to the bottom of the list and select the empty row at the bottom, then enter the required account in the Code field, select the Database and IC Contra Account in the right-hand pane and click Save.