My pokemon knows headbutt but when I want to use it there's a message that if I have a pokemon to get them our . I bought it also from the tutor but nothing happens and the same message appears. 

Greetings @Bobby2. 

 

In order to use headbutt your pokemon must have at least 150 or more happiness (The orange bar on your pokemon card). This goes as well for TM's like dig or rock smash (in certain locations). You can gain happiness mainly by winning wild fights, leveling up & walking with your pokemon. You can also boost the happiness yield times two by equipping a soothe bell on the desired pokemon. There is a free claimable one in Celadon office 3rd floor from the Tiggus NPC.

 

Be wary that fainting or trading your pokemon to another player will make him lose happiness. 

 

Here is a guide on happiness if you are looking for more information, let me know if anything is unclear. 
Have a nice day.