This code has an error but I do not know how to fix it
Clear[a, b, n, h, ETA, f, H, G, F1, F2, F3, k1, k2, k3, k4, l1, l2, \
l3, l4, m1, m2, m3, m4, Error]
a = 0;
b = 10;
n = 20;
h = (b - a)/n;
ETA = f = G = H = Error = Table[0, {n + 10}];
ETA[[1]] = 0;
f[[1]] = 0;
G[[1]] = 0;
F1[G_] := G;
F2[H_] := H;
F3[f_, H_] := -(1/2)*f*H;
Do[
H[[1]] = 0.01*j;
Print[Error[[j]] = 1 - G[[20]], " ", 0.01*j, " ", j];
Do[
k1 = h*F1[G[[i]]];
l1 = h*F2[H[[i]]];
m1 = h*F3[f[[i]], H[[i]]];
k2 = h*F1[G[[i]] + l1/2];
l2 = h*F2[H[[i]] + m1/2];
m2 = h*F3[f[[i]] + k1/2, H[[i]] + m1/2 ];
k3 = h*F1[G[[i]] + l2/2];
l3 = h*F2[H[[i]] + m2/2];
m3 = h*F3[f[[i]] + k2/2, H[[i]] + m2/2 ];
k4 = h*F1[G[[i]] + l3];
l4 = h*F2[H[[i]] + m3];
m4 = h*F3[f[[i]] + k3, H[[i]] + m3];
f[[i + 1]] = f[[i]] + (k1 + 2*k2 + 2*k3 + k4)/6;
G[[i + 1]] = G[[i]] + (l1 + 2*l2 + 2*l3 + l4)/6;
H[[i + 1]] = H[[i]] + (m1 + 2*m2 + 2*m3 + m4)/6;
ETA[[i + 1]] = a + i*h
, {i, n + 1}]
, {j, 1, 99}]
These are the mistakes
Set::partw: Part 31 of {1,0.91881,0.856369,0.804695,0.759696,0.719129,0.681693,0.64659,0.613306,0.581493,0.550904,0.52136,0.492723,0.464887,0.437766,0.411289,0.385399,0.360045,0.335185,0.310783,0.286806,0.263226,0.240017,0.217158,0.194628,0.172409,0.150485,0.12884,0.10746,0.0863339} does not exist.
Set::partw: Part 32 of {1,0.91881,0.856369,0.804695,0.759696,0.719129,0.681693,0.64659,0.613306,0.581493,0.550904,0.52136,0.492723,0.464887,0.437766,0.411289,0.385399,0.360045,0.335185,0.310783,0.286806,0.263226,0.240017,0.217158,0.194628,0.172409,0.150485,0.12884,0.10746,0.0863339} does not exist.
Set::partw: Part 33 of {1,0.91881,0.856369,0.804695,0.759696,0.719129,0.681693,0.64659,0.613306,0.581493,0.550904,0.52136,0.492723,0.464887,0.437766,0.411289,0.385399,0.360045,0.335185,0.310783,0.286806,0.263226,0.240017,0.217158,0.194628,0.172409,0.150485,0.12884,0.10746,0.0863339} does not exist.