Why i cant download the youtube

Unfortunately you have provided too little information for us to propose a specific solution. Describing what happens when you try, and the exact wording of any error messages, would greatly aid us in helping you. In the meantime review these suggestions and documents if you think they apply:

Read this document if you can’t download or update apps on your iPhone or iPad --> If you can’t download or update apps on your iPhone or iPad - Apple Support

If the first 5 don't help, use the 6th.

Another common reason is your device is too old to meet the system requirements for hte new YouTube app. If you cannot update the iOS on your device you will have to buy a newer device if you want to download it.