groupby doesnt work with sum() onlu cumsum()

Question

I'm trying to use grubby to do something similar to "SUMIF" but when I use "cumsum" it works normally but the "sum" appears as a result "NaN"

df1 = pd.read_excel(r'df directory')

df1['soma_lmg'] = df1.groupby('concat')['vl_is'].cumsum()[try with cumsum()]

if i try "sum()" i gave error[try with sum()]

sum() and cumsum() but doestwork

i want this: