Another advice that might help you (or not): tr. BUSD
If it doesn't help - then:
I think if you are not able to see tabs in TBI6 tr, then BDT (help.sap.com) Business Data Toolset was used. This tool helps to reconfigure screens, tabs, etc. For example - there are a lot of tabs, screens in tr. BP + there are several other transaction to configure business partner (for example TBI6 etc.)
So implementation team suggested to create all tabs that your company are using - on several custom tabs in tr. BP (this is usuall scenario).
So you have to check documentation, technical tasks. or check tr. BP for your business partner with different roles.
use tr. BUPT