Hello,
As soon as I checked the "pro forma" option in "configuration>sales>pro forma invoice" I got a pop-up message in all my old invoices (the new ones don't have this problem).
I tried to look for "sales condition" everywhere but I couldn't find it.
Here is a picture of the error message : https://ibb.co/yhhktp5
Do you know how to solve this problem please? Thank you very much.