[cosimobianchini]

If I did not misunderstand you should use your if statement within a loop on the internal cells of your volScalarField nut_. What the error is saying is that it makes no sense a statement like <= for a volScalarField in fact for some cells it will be > for others <.

In order to make it work write something like

forAll(nut_,cellI)
{
if(d_[cellI] <= (1.3*H_))
{
nut_[cellI] = a_*zh_*exp(-b_*d_[cellI]/H_)*mag(U_)*pow((d_[cellI]/H_),2);
}
else
{
nut_[cellI] = 0.16*Uh_*(d_[cellI] + z0_)/Foam::log((d_[cellI] + z0_)/z0_);
}
}

assuming that all other variables except for d_ and U_ are of type scalar this should work.
Another thing you should pay attention to is in the use of vector variables (I guessed U_ it is velocity vector) in a statement with scalars on the LHS, I added the operator mag(U_) to evaluate the magnitude of velocity but I'm not sure this is what you want. Change the operator accordingly to your needs but check that it returns a scalar.