Q: Could not Activate Mobile data

Hello Lampropoulou K,

Thank you for using Apple Support Communities. 

I understand from your post that you are receiving the error "PDP authentication failure" on your iPhone 6s. I know how important it is for you to understand and resolve these type of error messages quickly. Does this error occur when accessing the internet? 

The first step I recommend is to restart your iPhone by pressing the Home button and the Sleep/Wake button at the same time. The iPhone will shut off and then come back on again with the Apple logo on the screen. 

If the error returns, I recommend the following two articles: 

For a Wi-Fi internet connection: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

or 

For cellular data: If you can‘t connect to a cellular network or cellular data

Regards