You need to have the old owner remove his account. This helps prevent people from using stolen devices, but it can also be a pain when you get a used device if the other person didn’t take care of it before giving it to you. Here are the instructions from Apple on how to remove the device from the account, then you can activate it normally:
"Contact the previous owner and ask them to follow these steps to remove the device from their account:
1. Sign in to their iCloud account at www.icloud.com/find.
2. Click All Devices to open a list of devices linked to their account, then select the device to be removed. It should show a gray dot or the word “Offline” next to the device name.
3. Click "Remove from Account" to remove the device from their account.
After the device is removed from the previous owner’s account, turn the device off by pressing and holding the Sleep/Wake button located on the top-right side of the device. Then restart your device and proceed with device setup as you would normally"