there is a lock image on my "gifted" MacBook Air a1534 screen of

It is probably a black screen with a white LOCK icon in the middle of the screen and asking for a password. Correct? If my assumption is correct then your MacBook Air is asking for a firmware password. No one knows that password except the person who set it and that is probably the previous owner. Ask your son-in-law to get that Firmware Password for you. If he purchased the MacBook Air for you then he will need to contact the seller to get that password.

Firmware Passwords are set at the hardware level (For additional Security) and you will not be able to get past it without the password to install your macOS High Sierra.

Axel F.

If it is a firmware lock screen:

https://support.apple.com/en-us/HT204455

Then your son-in-law will need to disable the firmware password lock. Here is what the previous owner should have done before handing you the laptop:

https://support.apple.com/en-us/HT201065

You are welcome. Good Luck.

Axel F.