Locked iPhone 6

You can try using Find My Phone to Restore the phone.

Remove a device from Find My iPhone on iCloud.com - Apple Support

If necessary, if you are the original owner, take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store.

Genius Bar Reservation and Apple Support Options - Apple. https://www.apple.com/retail/geniusbar/

or contact Apple ID Support. They will need to see the original purchase receipt/paperwork also.

Apple ID - Official Apple Support. https://support.apple.com/apple-id

Contact Apple for help with Apple ID account security - Apple Support. https://support.apple.com/en-us/HT204169

Get Support https://getsupport.apple.com/. Work through the screens to get a chat option.

https://twitter.com/AppleSupport

Or fill out and submit iTunes Store - Contact Us - Official Apple Support https://support.apple.com/en-euro/itunes-contact The form is for iTunes, but it will get passed to the ID Support section.

On an iPhone 11, the app is just named "Find My". Look for this icon:

The "Find My iPhone" app is now only for older iPhones that do not support iOS 13.

Also, you need to restore the iPhone, through iTunes to remove the Activation Lock screen.

See here: Restore your iPhone, iPad, or iPod to factory settings - Apple Support

See these Apple reads on activation lock

Activation Lock for iPhone, iPad, and iPod touch https://support.apple.com/en-us/HT201365

Turn off Activation Lock https://support.apple.com/en-us/HT201441

If you can't activate your iPhone https://support.apple.com/en-us/HT201407

Go to here to choose a method of reaching Apple support (including by phone) https://getsupport.apple.com/?caller=kbase&PRKEYS=PF9&category_id=SC0068&symptom_id=20167

Note that Apple usually won’t unlock a phone without seeing its serial number on your purchase receipt. You may be able to take the receipt and the phone the to the genius bar (make an appointment https://www.apple.com/retail/geniusbar/) of a brick and mortar Apple Store for help in removing the lock.

Did you restore the iPhone after eliminating it from your devices?

Do you have access to a computer browser where you can log into iCloud.com/Find?