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)
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)
If you have followed all the steps and instructions and if you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.