The address might be referring to the tax setup at the company address. The object could be the sales part or the sales charge type. Can you please check the tax codes set there? 

The TAX is correctly configured on the customer's company address, and the sales part has no charges.

What are the boxes checked under Company > Tax Control > General > Tax Code Validations > Customer Tax ? 

 Here is the company’s Tax control confuguration.

From another thread I read that this could be a bug. See here: 

Project Invoice Create Correction Invoice - Identical tax code error | IFS Community

I'd tag the forum members there to see if they found a fix. 

Hello, ​@Faouziii  As ​@Asela Munasinghe  mentioned, we did have the problem when we installed Apps 10 UPD 13.  The workaround (go to the Project Transactions screen, select the transactions then RMB and create the correction transactions there) did work for us.  We have since upgraded to IFS Cloud (23.1.8) so I’m not sure in which Apps 10 version the problem is fixed.  

Hi ​@jtobin was the conclusion by IFS through the case G2352343 was to follow the workaround? If yes, there must be no bug fix created in the next UPD

Hello, 
I solved the issue by entering the tax code in the Non-Inventory Sales Part.

Thank you for your help.

​@Asela Munasinghe It was identified as a Sev3 and likely patched in a later update of Apps 10.  We didn’t install any Apps 10 updates after UPD13 because we upgraded to Cloud.  So I can’t confirm whether or not it was fixed in a later Apps 10 update.