I'm not a Thunar fan, so I removed it and installed Nautilus. When I try to open picture or folder I have this error
What should I do?
XFCE doesn't depend on Thunar, you can replace it with anything you want.
To change default file manager go to Configuration Manager, click on Preferred Aplications and in the Utilities tab you will be able to choose nautilus or right your own command
Thunar is very much integrated into XFCE, I don't know if removing it is even posible, and it seems it can't be replaced for some uses. How did you remove it? Please restore it (perhaps by reinstalling whatever your package manager reports as missing/damaged).
The error message indicates XFCE tried to run Thunar, and did not find it. This thread on XFCE states that you have to edit
[BTW, try to change the system to run in English when getting messages to show here, if I didn't understand a bit Italian, I'd be even more lost than you.]