Bootable Installer - error...

Hi all - I'm trying to salvage a friend's 2015 MacBook Air. 99% sure the HD is dead.

I'm looking at picking up a new SSD but the manufacturer says the old SSD needs to be at least 10.13 and it is currently running 10.10. I'm trying to do a bootable installer from my iMac (running 10.15) but when I try to download 10.15 it gets all the way to the end and I get:

I've tried Mojave as well but get the same result.

Not sure how to get the bootable installer over to the flash drive to install on the MacBook Air.

Any help appreciated!