Hello Sabella,
I have not worked too much with symbolic, but I think this approach may solve your problem.
In case you want to plot a specific range, fplot requires specifying the interval, not the range of values.
Rmax = 1410.34;
conc = 500e-9;
ka = 3.46e3;
kd = 1.46e-4;
t_vector = linspace(0, 1800, 600);
A = ka*conc*Rmax;
B = ka*conc;
C = kd;
syms y(t)
eqn = diff(y, t) == A - B*y - C*y;
cond = y(0) == 0;
s(t) = dsolve(eqn, cond);
fplot(@(t) s(t), [t_vector(1), t_vector(end)])
