Question: Why am I seeing "Unable to evaluate the function to numeric values in the region"?

Hello all. I'm using version 2021.2 to try to make some simple energy plots for my research. The math is pretty straightforward, but for some reason I cannot get the results to plot properly. My code is below:

restart;
A := 7.17;
B := 2.56*10^(-3);
C := 0.08*10^5;
_local(D);
1;
D := 0*10^(-6);
                           A := 7.17

                      B := 0.002560000000

                          C := 8000.00

                             D := 0

T_0 := 298; 

G0 := -71.398;

S0 := 45.106;

Hf := -57.95;

cp := A + B*T + C/T^2 + D(T)^2;             

`ΔH` := int(cp, T = T_0 .. T);

`ΔS` := int(cp/T, T = T_0 .. T);
           
G := -S0*T - T*`ΔS` + Hf + `ΔH`;

plot(G, T = T_0 .. T_max);

Which yields the following error message:

"Warning, unable to evaluate the function to numeric values in the region; see the plotting command's help page to ensure the calling sequence is correct"

I've looked through everything I can find on this issue, and I'm coming up empty. Anyone know what's happening here?