Each Apple device is linked/associated with an Apple ID of the person who did the setup.
Activation Lock prevents anyone who is not the owner from using this iPhone.
To unlock the Phone, you need the Apple ID and password of the previous owner, that were used during set up.
So you have to contact the previous owner.
The previous owner has to read -> this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch -> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
Then the previous owner has to remove the iPhone from his account, see this Apple article:
Check your Apple ID device list to see where you're signed in
Check your Apple ID device list to find where you're signed in - Apple Support
Then you can sign in with your own Apple ID.
See the following Apple articles:
Activation Lock for iPhone, iPad, and iPod touch
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
How to remove Activation Lock -> How to remove Activation Lock – Apple Support (UK)
Read this before you buy a newer iPhone.
You can also contact Apple support for help, -> Get Support (apple.com)
You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.
If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.
Once Apple receive your proof of ownership, Apple will further review your request and provide an update.
See also -> Keep your receipt after you buy an Apple device
Keep your receipt after you buy an Apple device – Apple Support (UK)
