With activation lock you have to either know the account information for removing the lock, or you have to be able to prove to Apple's satisfaction that you are the device owner.
Without the original receipt for the purchase of the device, you will find it very hard to succeed in the unlock request service, as you are discovering.
As for regaining access to the account information, that too is presenting difficulties. Try the directions in the article If you forgot the answers to your Apple ID security questions - Apple Support for resetting your security questions:
"Reset your security questions
1. Go to https://iforgot.apple.com/ .
2. Enter your Apple ID, then select Continue.
3. Choose the option to reset your security questions, then select Continue.
4. Enter your Apple ID password, then select Continue.
5. Follow the onscreen steps to verify your identity. The information you’re asked to provide may vary based on your account details and other factors. If you don't get any options to verify your identity, you can't reset your security questions at this time.
6. Choose new security questions and answers and select Continue."
Note that Apple labeled this article as "archived", which means it has not been updated since December, 2021, and may contain outdated information.
If you get a message about not having 'sufficient information', you will not be able to reset your security questions this way.
The article: About your Apple ID email addresses - Apple Support is updated and it implies you can use a rescue email (if you set one up -- if you did not, it is too late to do that now) to reset your questions: "If you ever forget your security questions, you can use your rescue email address to reset them. We also send any account security-related notifications to your rescue email address." How you do this exactly is not clear and my research into this reveals it may depend upon your region. Here is one possible way (from: https://www.alphr.com/we-do-not-have-sufficient-information-to-reset-your-security-questions/ ) but I have not tested it beyond the first step since I do not want to experiment with my account. ;-).
1. Visit this page and enter your Apple ID -->.https://iforgot.apple.com/password/verify/appleid
2. Enter your rescue email address and have a verification email sent there to reset your password.
3. Follow the link in the email to reset.
If you cannot get any of the above to help with resetting the security questions then realize that several years ago Apple transitioned to using to two factor authentication instead of security questions. Apple has since been encouraging users to switch to the newer security system and your only recourse may now be to try to change to two factor authentication as outlined in this article --> Two-factor authentication for Apple ID - Apple Support Follow the instructions in the section "Turn on two-factor authentication for your Apple ID"
If you can neither reset the questions, nor switch to two-factor authentication, you may well have lost any access to that account and also the device itself.
If you wish to speak with Apple about this, contact with Apple ID account security using the information in this document --> Contact Apple for help with Apple ID account security - Apple Support
