Activation lock

The previous seller has removed the credentials from his icloud account https://support.apple.com/en-in/HT201065

And then son took over the Mac possession that's the reason he created his own Apple ID and password and signed in successfully , if Apple ID password is forgotten , use another browser and go to iforgot.apple.com and reset the Apple ID password https://support.apple.com/en-us/HT201487

The passwords and other credentials were entered multiple times the server assumed some phishy attempt and locked it , try to reset password after twenty four hours https://support.apple.com/en-in/HT204106 ( it all depends how was the account created with secondary authentication in which three answers for security questions , rescue mail address and its password , date of birth is added or with two factor authentication .

Then see the steps https://support.apple.com/guide/findmy-mac/activathttps://discussions.apple.com/docs/DOC-12712

https://support.apple.com/en-us/HT208987

https://support.apple.com/en-us/HT201441

Come to your your original Mac , in recovery mode reset admin password https://support.apple.com/en-in/HT202860

See the upcoming steps https://discussions.apple.com/docs/DOC-12712

If you don't have previous seller Apple ID and password and no contact with that person , then it seems impossible .

Apple support will help only if you are the first owner of the Mac and for your Apple ID account data ( you can still consult senior apple care advisors and request them for a help ) https://support.apple.com/en-in/HT201232

Was this refurbished by an authorized reseller or Apple itself?

Read this link, particularly the stuff at the end:

About Activation Lock on your Mac - https://support.apple.com/HT208987

"What to do if your Mac starts up to an Activate Mac window

If you've purchased a Mac from someone other than Apple or an authorized Apple reseller and the device starts up to an Activate Mac window, the device is still linked to the previous owner's account. You'll need to ask them to remove the device from their account."

Find out from Apple what proof of purchase they need in order to believe you. If they really won't believe anybody except an authorized reseller or Apple itself you may be in a spot. It would be really unfortunate but I bet Apple hears all kinds of stories, including faked receipts or receipts from shady businesses, trying to get things unlocked and has to really get solid proof something isn't stolen.

Jut so you know, we are users like you, not Apple itself, so we can only provide advice.

Go to iforgot.apple.com and type only apple id in the field .

Reset options - choose how would you like to reset your password ( a )

Click on reset through mail and click on ok .

Then open your rescue mail with web browser ( server account ) , open mail > click on reset password > two fields will be shown .

( a ) new password …..

( b ) verify password ….

Enter the credentials , and click on reset password , the password is changed .

If you click on - answer security questions - click on the right arrow .

Date of birth , verify your date of birth to continue > fill up the credentials > click on next and same pop up window will appear .

New password ….

Verify password ….

Fill up the credentials and click on reset password , the password is changed .

You had not added the rescue mail address / the date of birth is incorrect so setting the password will fail .

What about activation lock is it removed from the Mac , it needs Apple ID and password what is one password ?

You can return the Mac to the store seller , and explain what is happening in Mac as it seems you will unable to use it .

See the article https://support.apple.com/en-in/HT201065

You can consult senior apple advisors in apple support before making any big steps .