Not able to select the Business unit in the drop down of Manage Tax code

We have a requirement to club two WHT codes in a group so that it can be assigned to one single invoice line.
We are following this note and all the setup as per this note are there.

https://support.oracle.com/support/?kmContentId=2166549&page=sptemplate&sptemplate=km-article

But still when we are trying to select the Business unit under the task "Manage Tax codes" and "Manage Withholding Tax classification", we are not able to select the correct BU. I have reviewed some of the threads on this topic, and ensure all the setups but nothing is missing. Any suggestions?