I have, on the one hand the measurements Y(s)=Y, and on the otrher hand the model Y(m)=B+D+E. That is, I have the following equation

Y(s) = Y(m)

Y = B + D + E

Then, its combined typical uncertainty is

u2(A) = C2(B)u2(B) + C2(D)u2(D) + C2(E)u2(E)

But, I want to get the uncertainty of D, and in this case, and applying fundamental algebra, we can obtain

C2(D)u2(D) = u2(A) - C2(B)u2(B) - C2(E)u2(E)

and then

u2(D) < 0

and these are complex uncertainties.

What is the physical meaning?

is this the correct way to work to get the uncertainty of D?

Thank you!!