1. Go to My PlayStation 3 > System Settings.
2. Select Game Data Utility > Delete Data > YouTube.
3. Delete all the YouTube data. After the deletion, exit to the main interface.
4. Go to the PlayStation Store.
5. Log in to your account and search for the YouTube app.
6. Install the app again and try to activate it.
Activating with a Different Account:
If the above method doesn't work, try using a different PlayStation account to activate the app.
Resetting the License:
1. Go to Home > Settings > System > Restore Default Settings.
2. Select Restore Default Settings.
3. Go to the PlayStation Store and log in to your account.
4. Find the YouTube app and install it.
5. Open the YouTube app and try activating it.
Activating in Safe Mode:
1. Turn off the PS3.
2. Press and hold the Power button until you hear a beep. Release the button after the second beep.
3. Connect a controller to the PS3 with a USB cable.
4. Press the PS button on the controller.
5. Select Restore File System from the Safe Mode menu.
6. Once the file system restoration is complete, restart the PS3.
7. Open the YouTube app and activate it.