Please tell me how can i remove activation lock from iphone 6plus without previous owner because the iphone and apple id owner is not contacting to me and I am stuck on activation lock screen. Please answer me.

Before you buy an iPhone, iPad or iPod touch from someone other than Apple or an authorised Apple reseller, make sure that the device has been erased and is no longer linked to the previous owner’s account. 

If you see the Activation Lock screen, the device is still linked to the previous owner’s account. If they’re close by, ask them to enter their Apple ID and password on the Activation Lock screen and remove the device from their account.

If the previous owner isn’t present, contact them and ask them to follow these steps:

1. Sign in to iCloud.com with their Apple ID.
2. Go to Find My iPhone.
3. Click All Devices at the top of the screen.
4. Select the device that you want to remove from iCloud.
5. Click Erase [device]. Select Next until the device has been erased. 
6. Click Remove from Account.

After the previous owner has removed the device from their account, turn off the device and then turn it back on to begin the setup process.

If you can't reach the previous owner and have them remove the device from their account, the device is worthless. Apple won't unlock it for you.