This is because the analysis code is marked as Inactive within Dimensions. You can either choose a different analysis code that isn't inactive or to activate the analysis code:
Log in to Dimensions
On the Creditors tab, in the top left, click Analysis.
Click Find.
Enter the inactive analysis code
Press the Enter key.
Click Edit.
Click Info.
Clear the Inactive check box.
Click Save. This analysis code is now available for use when posting from FocalPoint.
