Ok I solved it:
I was getting an error because when I first added the frontend form I had attributed to a LABEL a text TYPE and set it as a REQUIRED validation condition.
When I later changed it to ACF field the REQUIRED validation (which is not visible when you change to ACF field) stayed on creating a conflict.
Hurray
Also I tired, out of the 2 acf fields I took one away and the form updates my posts.
