If the previous owner could not find how to disable?

You can't disable activation lock unless you can contact the previous owner. See:

Turn off Find My iPhone Activation Lock - Apple Support

Activation lock is an anti-theft feature. If we knew how to bypass it, it wouldn't be much of a deterrent. (I'm not saying you are a thief.) The device is useless to you. Try to get your money back if you can. Otherwise turn it over to the authorities.

As you say 'previous owner' in your first post I assume you bought this phone used. The fact that it is yours now doesn't alter the fact that if it has been locked by the previous owner only he can unlock it. There is no way round this. If you can't contact him you will not be able to use the phone.

You said 'previous owner' in your original post. Did you buy it new from a shop? Or did you buy it from someone who had been using it previously? If you bought it new and have simply forgotten the password you will have to erase the phone - please see https://support.apple.com/HT204306

if you bought it from someone else, not new, then only they can unlock it.

Apple ID - How to find your Apple ID

Apple ID - Find

Enter your Apple ID and click Forgot Password,

Apple ID - Manage