Why can't you do a voltage divider for a transmission line?
One way to calculate Zin as the input impedance to the line with load is
$$ Z_{in} = Z_0 (\frac{Z_L + j Z_0 \tan{\beta l}}{Z_0 + j Z_L \tan{\beta l}}) $$
which is a non-linear function of \$Z_L\$ and \$Z_0\$, and wavelength and line length.

The whole transmission line plus the load can therefore be modeled as one lumped impedance as \$Z_{in}\$. But you cannot use this model yet isolate \$Z_L\$ out to form a voltage divider, which would require you to assume \$Z_{in} = Z_0 + Z_L\$ (which is wrong).