Why I can’t download YouTube app ?

Then you need to install iOS 12.5.5 on your IPhone 6 plus

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.5

• If you have an iPhone 6S or newer, you can update to 15.0.1

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch -

https://support.apple.com/en-us/HT204204