Warning: Matrix is close to singular or badly scaled.

a=cos(pi/6); 
b=cos(pi/3); 
%  Fab Fag Fbc Fbg Fcd Fcf Fcg Fde Fdf Ffe Fgf Ax Ay Ey
T=[ b   1   0   0   0   0   0   0   0   0   0   1  0  0 ;% Ax      
    a   0   0   0   0   0   0   0   0   0   0   0  1  0 ;% Ay
   -b   0   1   b   0   0   0   0   0   0   0   0  0  0 ;% Bx
   -a   0   0   a   0   0   0   0   0   0   0   0  0  0 ;% By
    0   0  -1   0   1   b   b   0   0   0   0   0  0  0 ;% Cx
    0   0   0   0   0   a   a   0   0   0   0   0  0  0 ;% Cy
    0   0   0   0  -1   0   0   b   b   0   0   0  0  0 ;% Dx
    0   0   0   0   0   0   0   a   a   0   0   0  0  0 ;% Dy
    0   0   0   0   0   0   0  -b   0  -1   0   0  0  0 ;% Ex
    0   0   0   0   0   0   0  -a   0   0   0   0  0  1 ;% Ey
    0   0   0   0   0  -b   0   0  -b   1  -1   0  0  0 ;% Fx
    0   0   0   0   0  -a   0   0  -a   0   0   0  0  0 ;% Fy
    0  -1   0  -b   0   0  -b   0   0   0   1   0  0  0 ;% Gx
    0   0   0  -a   0   0  -a   0   0   0   0   0  0  0];% Gy
F=[ 0; 0;0;0;0; 0;0; 0;0; 0;0;-8;0;-6 ]; % Applied forces, as shown on the truss
M=inv(T)*(-F)

[darova]

Its because of inv(T). It doesn't exist. Are you sure your stiffnes matrix is correct?