Hey there, kyriakos163.
Welcome to the Apple Support Communities! Thanks for posting about not being able to get connected to the App Store to complete app downloads. We'd be glad to see what we can do to help you out.
Let's look into a few things together. When was the last time you were able to sign into the App Store successfully? Do you use the same Apple ID for iCloud that you do for the App Store? We ask because they can be different. Does this same Apple ID work to successfully sign in to the iCloud site or the Apple ID site?
Something else you may want to do during troubleshooting of this issue is turn off Low Power Mode, as that can affect different aspects of your iPhone, and we want to remove as many potential players as we can during this process.
Let us know what you find out.
Take care.