This account is locked. This account is locked and cannot be used.

As you probably realize now, to unlock the phone (as explained in: How to remove Activation Lock - Apple Support) you need either:

1) The account used to set it up, to which you no longer have access. Another post says this must be the "previous owner", but the contributor did not understand that you are the "previous owner" and the iPhone is locked to the account you created and is now permanently disabled, not to that of the person who sold you the phone.

or:

2) Have Apple reset the device which typically requires the original receipt from Apple or and Apple authorized retailer. You unfortunately also do not have that. This is a disadvantage of purchasing a used device where the seller has not provided the original receipt.

The people on this forum are users like yourself, not Apple, so we can't do anything about the account. According to various posts, Apple may permanently lock accounts for other reasons (very broadly outlined in section VII-B of Legal - iCloud - Apple but Apple won't be more specific than that. You, or your relative are likely doing something Apple is interpreting as account misuse. You will have to see if your activities could be construed—rightly or wrongly—as meeting any of Apple's conditions in the support article.

For future reference:

Keep your receipt after you buy an Apple device - Apple Support

The All Too Common SAD Reality of Buying … - Apple Community

Sorry, but without any proof of purchase, the official Apple Support can not disable the Activation Lock, only the previous owner could do that.

How to remove Activation Lock - Apple Support