Hello Alex,
You will tend to get a quicker response if you post a bit of code instead of a notebook.
In any case, I took a quick look at your code. You had values of g inside the inner Numerical integrate. You need to make that explicit, with something like B[T,g] instead of B[T]. If it inside a numerical function, then it is better to do this:
B[T_,?NumericQ, g_?NumericQ]:= B[J_, g_] :=
(2/(1 + nIntegrate[ Exp[-B[J]]*(b*h/G*(1 - c*g) + b*(1 - h/G)*(1 - c*g)*g + d*x/K), {J, T, 505 - T}]))
I put a dummy function in there "nIntegrate" so you can make sure that the integrand evaluates to a numeric quantify for values of J. If it does, replace the nIntegrate with NIntegrate.
Also, note that capital K is a symbol already defined in the WL.
Your plot will take a long time as you have to do nested numerical integrals.