I assume you mean after the first part of the fight and then a cutscene occurs, and then you go and climb up the left side?

If you're unable to climb it, despite having the climb symbol, I don't know what to tell you.

yes exactly i can't climb :( i guess is the end of my story
I even reloaded a save before that and i get stuck the same

EDIT: ok i reloaded an old save did all the dragon thing on the stairs but this time didn't hit his head , the event on the toweer was different this time and could finish it phewww

Here is the actual solution for this:

After you talk to Sile and decide what to do with her, the game autosaves. Load this earlier autosave. When (or before) the dragon pops his head through the wall, cast Quen, then go over and hit the dragon ONCE (only once) on the head, then retreat. He will then use his claws to WIDEN the ledge you eventually reach.

Should you instead go up and hit the dragon multiple times before he does this, he will never widen the ledge and instead you'll get a shorter, bugged ledge that you cannot climb up from.

After he widens the ledge, then hit him a few more times and you'll get the cutscene where you jump to the ledge. Then game will then autosave again, and you should be able to climb that first ledge.

You can tell if you're on a bugged ledge if you cannot walk backwards (away from the Climb) very far. On a non-bugged ledge (that has been widened by the dragon) you can walk all the way around in the opposite direction.

I did NOT need to reload to before the tower climb (and meeting the dragon) in order to fix this. For what it is worth, I am running an unpatched Witcher 2 (initial GOG version). I have no idea if the patch fixes this bug.

-Mark