Hello, Dimitrios_Tsiplakides!
Thanks for using Apple Support Communities, it's great to have you here. Reading your message, I understand that you'd like to get the 'For You' tab in Photos to work as expected. That makes sense to me and I'm happy to help.
Just to be clear, when you received the message that recommended connecting your phone to power, were you able to do that? If you just updated to iOS 12 for the first time, it's possible that the device is attempting to gather data and prepare that For You tab for regular use. To make sure that it's able to do so uninterrupted, I'd recommend connecting it to power and giving it some time to finish up.
I hope you find this helpful.
Best regards.