Hello, my bike has the same problem in this video 
https://m.youtube.com/watch?v=jSqGtpsE8L8

It start if i put it in motion to push and everything is okay. 
Could it be the starter relay? 
Thank you

If that is what you are seeing and hearing, your sprag clutch is not engaging.  This is a fairly common problem caused by owners who don't change the oil frequently enough and there are many threads here addressing it. 

There are several ways to treat it, that involve cleaning the sprag itself.  A search of the threads where these techniques are discussed will let you decide which way you want to go. 

The problem is mainly caused by dirt in the pawls of the sprag that prevent them from "flopping over" to engage. 

I corrected the problem in one of my bikes by draining a quart of oil and replacing it with a cleaner like Seafoam and push starting the engine.  Go for a ride for about an hour being very easy on the engine(the oil has been diluted) and drain the oil when you get back.

Then refill with a good 15W40 oil made for diesel engines(Shell Rotella T6 is my favorite).  This has a lot more detergent for cleaning up the soot that gets in those engines.  Run for a normal oil cycle(I go about 5000 miles) and go back to your usual oil.

If you have trouble push starting, there is a lot of information on cleaning the sprag with spray to get the engine running.  Still, when you do, you want to do the Seafoam flush and diesel refill.

In the meantime, try having the bike in gear and rocking it backwards before attempting a start. Sometimes that will help. And if that does work then it's pretty good confirmation that it is a dirty sticking starter sprag that's your issue.

K bikes are pretty easy to push start. TURN THE IGNITION ON. Put it in SECOND gear. Have the clutch pulled in. If you're on a hill then just roll until you're going a few MPH(kph) and then the let clutch out somewhat quickly but gently. Or have somebody push and then let the clutch out when you get rolling.  Once you it does begin to start pull the clutch back in, rev it a little to confirm that it's running and then re-engage the clutch normally.

You don't to be going extremely fast to push start a K. A jogging pace should suffice.