Option key won't load boot menu 2009 macbook air

I just got a macbook pro and want to sell my macbook air. I want to erase all data from the air before I sell it. I used "command+R" during startup to do this. However, when I actually try to erase the disk is says something like, "operation failed, can't unmount disk"

Based on my research it sounded like I needed an external boot-able drive, so I made one using terminal following the commands from apple's page. On my new macbook pro I can hold option during boot and am brought to a screen where I can select the new boot-able external drive I just made, or the internal drive on the macbook pro.

Unfortunately, when I do the same thing from my macbook air it just loads from the macbook air's hard drive.

I attempted to reset NVRAM by holding "command+option+R+P", which brought me to the same screen that holding "command+R" brings me to. I have tried everything I can find online. It sounds like lots of people are having this issue based on my search results.

Does anyone know how to get my Mid 2009 Macbook Air to load the boot manager? Again, I have tried holding option during boot... many times and starting at many points including prior to pressing power button, while pressing power button, just after pressing power button, right when I hear the startup sound, etc.

Thanks for any help.

PS: any other suggestions as to how to erase my disk and load a fresh copy of macOS are also appreciated.