When I ski, at the end of the turn, I'm not able to get good grip at the end of the turn and the tail washout, why does this happen?
There are several factors that can contribute to ski not having grip at the end of the turn and the tails washout.

First thing is to make sure your ski edges are sharp, if they are not sharp, you can visit your local ski shop and have the ski shop tune your ski edges.

Once you've made sure your ski edges are sharp, then this might be caused by either incorrect fore-aft balance and/or lateral balance. 

If you are too forward at the end of the turn, meaning you feel more pressure on the ball of your foot, then that will cause the tail of the skis to not be able to bite into the snow and cause it to skid out. At the end of the turn, you want to bring your feet forward and aim to have at least half of the pressure just in front of the heel of the foot, this will allow the edge along the tail of the ski to bite into the snow.

If you have too much pressure on the uphill foot, for example, you are leaning up the hill or transferred your weight too early onto the new downhill foot, that can also cause your current downhill foot to not get enough grip and washout. Try have majority of your weight on your downhill foot until the turn is finished before transferring majority of the weight to the new downhill foot.

It will also be helpful to find a good ski instructor at your local resort or online to look at your skiing and provide feedback.