This iCloud is not mine
I bought a used iPhone 6s, and after I created a software, I found that it had iCloud, it was not mine, and I could not open it
The only way to unlock the device is to have the owner login with their credentials to remove their Apple ID and the Activation Lock. If you cannot find the owner, the device is unusable. Do consider recycling it if possible - Recycling - Apple (ME).