Light Speed is said to be constant irrespective of relative motion of observer.

Moon is about 1.3 light seconds away from us. Suppose moon is looking like a clock such that moon clock is behind your watch by 1.3 seconds.

Now you start moving towards moon with a normal speed of 100 km/hr. Moon is 384400 KM away so your journey will take about 160 days.

Normal speed is assumed so that there should be no alleged effect of time dilation due to ultra high speed.

So after 160 days you will be on moon and what will be the difference in time of your watch and moon clock…???

There will be no difference. Means that due to your relative motion, light of moon clock has traveled up to you at speed c+100 km/hr i.e. with speed c+v.

If you are still reading moon clock as 1.3 seconds behind your watch then it means that from an ordinary distance, light is taking 1.3 seconds to reach you so again constant speed of light postulate is disproved.

Above is only a thought experiment. Now we do a real experiment. So now moon is looking like normal moon and not like a clock. And from earth we see moon which is 1.3 seconds younger than its actual age.

Now we do not suppose, we actually start moving towards moon at normal speed of 100 km/hr. And after 160 days we reach at the surface of moon.

By standing on surface of moon we are looking at moon which is not younger by 1.3 seconds but are looking at moon that exists right now. It means that during whole of our journey we had been receiving light of moon with c+v formula.

If by standing on surface of moon we are watching moon which is 1.3 seconds younger than its actual age, it means that light is taking extra 1.3 seconds to reach our eyes and again constant speed of light postulate is disproved.