I tried without the aggregation but it didn't work. I am refering to the total of Nbr de cas Hors delais 204 and the Total of Nbr de cas dans les delais 2816
I think you use if in your expression : if(condition,1)
so in expression % you can't use name of field, but repeat the formula :
sum(if( ,1)) / (sum( ) + sum())
and in this expression : check 'total of expression' and not 'sum of rows'
without the expression difficult to say what's wrong in expression !