See the error in the screenshot.
I use linux mint 19 (based on ubuntu 18.04)
You need to explain what you are trying to do when you get this message.
ah sorry, i wrote it in the topic title.
This error occurs when editing or creating a supplier.
Yes, I could tell that from the screen shot. But when, exactly, does it happen? Obviously, you were able to enter some information before the message appeared. And what version of the software are you using?