I didn’t do it with the editor, but following this article.
Well the template editor is a debugging tool. It is designed to help with such problems, so give it a try.
After a few attempts I solved it, all I had to do was put the double quotes. For a stupid thing, I wasted a lot of time.
temperature: "{{ state_attr('climate.split_salone' , 'current_temperature') | float | round(0) + 1 }}"
Thanks anyway.