See Turn off Activation Lock https://support.apple.com/en-us/HT201441.
Since you acquired your device second hand form an individual, and if you can contact the prior owner (which your should be able to do), you will need to obtain the needed password from them or have them sign out of iCloud; erase the device; and remove it from their device list. Alternatively, if the prior owner has their purchase receipt for the device from an authorized Apple seller with the device’s serial number on it, then they, along with you, may be able to persuade Apple support to remove the activation lock. Use the support options below. Alternatively, if an Apple Store is open in your area, they, along with you, can go there to deal with the problem. In this case, you can make a genius-bar appointment (https://www.apple.com/retail/geniusbar/) if desired.
If none of the above work, then the device is useless to you.
Apple ID Support https://support.apple.com/apple-id
Apple Support Twitter https://twitter.com/AppleSupport
Apple ID account security for country-specific andregion-specific help https://support.apple.com/en-us/HT204169
Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.