If you cannot log into YouTube successfully on the TV, here are some methods to do the troubleshooting for your reference:
1. Make sure the TV has internet access.
2. Clean the cache of the APP on the TV and try again. The cache could be cleaned on the Settings --> Apps --> Select the App YouTube --> Clean Cache:
3. If the TV has internet access, please check if there is a new version of software updated online or not.
Press the Settings 
button on the TV remote, then select Settingsà System à About à System Update à Network Update:
If the issue cannot be resolved, please feel free to call the TCL Helpline for further help.
