I bought an iPhone, but here someone else’s Apple ID

you will need to contact the previous owner to have them log out of iCloud or go to https://www.icloud.com/find/ My Phone and erase the device, then remove it from their account. If you can’t do that, the device is useless. Apple won't unlock it.

Turn off Activation Lock - Apple Support.      

Is your device activation locked? If yes see Turn off Activation  Lock https://support.apple.com/en-us/HT201441. Sometimes Apple Support can help you with activation lock. But, note that if 

(1) you can't use the instructions in the above document to. remove the activation lock;

(2) you, or the person you acquired your device from, doesn't have the receipt from an authorized Apple seller for the purchase your device with the device’s serial number on it;

 (3) you don't have a court order instructing Apple to assist in the provision of access to a decedent’s information from the deceased person's accounts; or

(4) you can't obtain the needed password from the person you acquired the device from, 

then the device is useless to you.

If you want to contact Apple support to deal with your problem, see the support links below. Alternatively, if an Apple Store is open in your area, then you can go there to deal with your problem. In this case, you can make a genius-bar appointment (https://www.apple.com/retail/geniusbar/) if you want.

Get a call from Apple support to help solve your problem using this

link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

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

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.