I can't change the volume of my music with my keyboard ?

Hello,

Im on the last version of Yosemite

And today i was listening music on iTunes and i discovered that I can't change the volume of my music with my keyboard,

There is an image that you have when you change the volume appear but there is a prohibition sign in this image as you can see under

so after I tried on spotify, but this is the same...

can you help me please ?

Thank you !!!

PS : im french, sorry for my bad english...