Why can't I get in my phone after I reset it?

You cannot get into your phone after you reset it because you activated (Or whoever set it up) the Factory Data Reset Protection (FRP).

You (or whoever) would have set up an email account and password on Gmail. Just go through the prompts given to reset the password. You can alternatively log into Gmail directly through your computer and reset it there as well.

You should know the email it was set up on, the Password, a past password you can remember, another email address or phone number that was set up on it. They give you several different options to help you access this information unless you are not the original owner and don't have this information.

This is for protecting the device from being lost or stolen.

The only other way I know of is to either contact the original owner or pay a service to unlock it for you which requires a "Type" of rooting the phone.