Client has just installed the OneStream Excel add-in but they have a conflicting add-in already installed. Do you have any advice?

There are a number of knowledge base articles related to the Excel add-in (Category: Excel Add-in). Without details on the error message being received, we would suggest looking through these articles.    
If the KB articles do not assist, raise a ticket with support. 

Source:  Office Hours 2020-07-02