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You have to access to the tensor components by directly passing the indices to the operator [], not via a string (as @tolga 's comment pointed out, string access shall be used outside loops). Moreover, you have to substitute Id = by Id +=. So the code should be something like

Id = 0
for i in range(4):
    for j in range(4):
        for k in range(4):
            for l in range(4):
                for m in range(4):
                    for n in range(4):
                        Id += R_down[i,j,k,l]*dual[i,j,m,n]*R_up[k,l,m,n]