Question: Error, (in dsolve/numeric/bvp/convertsys) unable to convert to an explicit first-order system

I am trying to solve a system of 4 odes by using the following program:

with(plots):

Pr:=6.2:Ha:=10^(-12):Nt:=0.1:Nb:=0.1:Nr:=0.1:Gr:=0.1:Ec:=0.1:Rb:=0.1:Lb:=1:Pe:=1:tw:=0.2:Le:=5:

fcns:={f(y),T(y),S(y),G(y)}:

sys:=diff(f(y),y$3)+f(y)*diff(f(y),y$2)-diff(f(y),y)*diff(f(y),y)-Ha*diff(f(y),y)+Gr*(T(y)-Nr*S(y)-Rb*G(y))=0,

diff(T(y),y$2)+Pr*f(y)*diff(T(y),y)+Nb*diff(S(y),y)*diff(T(y),y)+Nt*diff(T(y),y$2)*diff(T(y),y$2)+Pr*Ec*(diff(f(y),y$2)*diff(f(y),y$2)+Ha*diff(f(y),y)*diff(f(y),y))=0,

diff(S(y),y$2)+Le*f(y)*S(y)+diff(T(y),y$2)*(Nt/Nb)=0,

diff(G(y),y$2)+Lb*f(y)*diff(G(y),y)-Pe*(diff(S(y),y$2)*G(y)+diff(S(y),y)*diff(G(y),y)+ga*diff(S(y),y$2))=0:

p1:=dsolve({sys,f(0)=tw,D(f)(0)=1,T(0)=1,S(0)=1,G(0)=1,D(f
(6)=0,T(6)=0,S(6)=0,G(6)=0},fcns,type=numeric,method=bvp[midrich],abserr=1e-10);

The error displayed is

Error, (in dsolve/numeric/bvp/convertsys) unable to convert to an explicit first-order system,

Please help me.