I’m not familiar with that error (never encountered it as you describe) but this might be what you want:
for k1 = 1:length(axial_induction)
if axial_induction(k1)<0.4;
C_T(k1)=(sigma_local .* ((1-axial_induction(k1)).^2) .* ((Cl.*cos(relative_wind))+(Cd.*sin(relative_wind)))) ./ ((sin(relative_wind)).^2);
else
C_T(k1)=(8/9).*(4.*F-(40/9)).*axial_induction(k1)+((50/9)-4.*F).*(axial_induction(k1).^2);
end
end
A guess here, but that is how I would do it.