AppStore not working on my old OS 10.9.5 so I can't download Big Sur

Title says it all really. But just to be clear, I am trying to download Big sur after many years of not realising that I could. When I try to download it, it asks for me to log in. I do so, and it sends a message to my cheap and basic smartphone with the 5 or 6 digit number to access the download, but i can't input it because there is no area to input it. I just get a red text saying that I need a confirmation code (which i received) and the password to enter. I tried it with my friends iphone and their account and they receive the code on their phone, but there is no where to input the digits on the mac. Can I somehow turn this function off maybe in order to bypass it?

I think my problem is that the os x 10.9.5 does not have this confirmation code window, so it can't go forward with the process. I was wondering if i can download Big Sur any other way without having to access the App Store?

Thank you in advance.

K