Thanks for your precious help.
I solved the problem as I define result_if_false in a if function beside using multiples if functions.
Code:
{if(sum(Minutage_SQ001.NAOK,Minutage_SQ002.NAOK,Minutage_SQ003.NAOK,Minutage_SQ004.NAOK,Minutage_SQ005.NAOK,Minutage_SQ006.NAOK,Minutage_SQ007.NAOK,Minutage_SQ008.NAOK,Minutage_SQ009.NAOK,Minutage_SQ010.NAOK,Minutage_SQ011.NAOK,Minutage_SQ012.NAOK)==(sum(Temps_H1.NAOK*60,Temps_M1.NAOK)),"Excellent! Your timing is ok.",join("Attention, your timing is'nt right. You have a total of ",(sum(Minutage_SQ001.NAOK,Minutage_SQ002.NAOK,Minutage_SQ003.NAOK,Minutage_SQ004.NAOK,Minutage_SQ005.NAOK,Minutage_SQ006.NAOK,Minutage_SQ007.NAOK,Minutage_SQ008.NAOK,Minutage_SQ009.NAOK,Minutage_SQ010.NAOK,Minutage_SQ011.NAOK,Minutage_SQ012.NAOK))," minutes, when the lenght of your workshop is ",sum(Temps_H1.NAOK*60,Temps_M1.NAOK)," minutes."))}