NDSolve::bdord: Boundary condition (u^(0,1,0))[x,0,t] should have derivatives of order lower than the differential order of the partial differential equation.

P[x_, y_, t_] = e[x, y, t]/(γ - 1) ;
e[x_, y_, t_] = (γ - 1) ρ[x, y, t]/(μ mu ) kb T[x, y, t];
cp = 5/2 kb/(μ mu);
Rgas = 8.3144598;
cv = 5/2 kb/(μ mu) - Rgas;
γ = cp/cv;
g = 28.02*9.81;
μ = 0.6163328197226503`;
mu = 1.66053904*10^-27;
kb = 1.38064852*10^-23;
sol1 = NDSolve[{
D[ρ[x, y, t]*u[x, y, t],
 t] == -D[ρ[x, y, t]*u[x, y, t]*u[x, y, t] + P[x, y, t], x] -
  D[ρ[x, y, t]*u[x, y, t]*
    v[x, y, t],
  y],
D[ρ[x, y, t]*v[x, y, t],
 t] == -D[ρ[x, y, t]*v[x, y, t]*
     u[x, y, t],
   y] - D[ρ[x, y, t]*v[x, y, t]*v[x, y, t] + P[x, y, t], y] +
  g ρ[x, y, t],
D[ρ[x, y, t], t] == -D[ρ[x, y, t]*u[x, y, t], x] -
 D[ρ[x, y, t]*v[x, y, t], y],
D[e[x, y, t], t] == -D[u[x, y, t]*e[x, y, t], x] -
 D[v[x, y, t]*e[x, y, t], y] -
 P[x, y, t]*(D[u[x, y, t], x] - D[v[x, y, t], y]),

v[0, y, t] == v[12*10^6, y, t],
u[0, y, t] == u[12*10^6, y, t],
T[0, y, t] == T[12*10^6, y, t],
ρ[0, y, t] == ρ[12*10^6, y, t],

e[x, 0, t] == 3.83767261162,
v[x, 4000000, t] == 0,
v[x, 0, t] == 0,
(D[u[x, y, t], y] /. y -> 0) == 0,
(D[u[x, y, t], y] /. y -> 4000000) == 0,

v[x, y, 0] == 0,
u[x, y, 0] == 0,
T[x, y, 0] == 5770 + 0.00835414960707927 y,
ρ[x, y, 0] ==
1.42*10^-7*1.408*10^3 + 7.3561137493644*10^-10 y
},
{u, v, T, ρ}, {x, 0, 12000000}, {y, 0, 4000000}, {t, 0, 100}]