The PayU wasn't originally developed by us so the best solution will probably be to directly ask it to the PayU developers.
Also :
Daniel suggested that, there is a problem on your Hikashop configiration/settings. I couldnt find where is the mistake. Could you please help me ?
If you are able to select the PayU on your main page it means that you configured your Hikashop well, the fact that it gives an invalid signature error means that the error is coming from the PayU payment plugin OR the way you configured your PayU plugin.