I've seen this before. From memory it was an issue for a lot of people when trying to install El Capitan from an external USB drive, rather than from the internal boot drive. For some users it would resolve itself by leaving it for a few hours, but obviously you've already tried that.
There's a couple of workarounds I've seen, briefly summarised below:
- Download/install the Google Noto fonts - This was a simple fix for many users. Basically you download the Google Noto fonts and then manually install them onto the USB drive at the following location:
\System\Library\Fonts. I don't recall now, but you may need to actually create the Fonts folder in that location. Anyway, once installed, try installing again. (Note: you may need to
- Install to an external drive first - This one takes a lot longer to do. Basically you install El Capitan onto an external drive first and then create an exact clone of it onto your internal drive. You can use software such as Carbon Copy Cloner or SuperDuper! to do this.
Another option, although you don't specify which model of MacBook Air you bought, is to try booting up with the shiftoptioncommandR keys held down. This combination should reinstall the macOS version that shipped with your Mac, or the version closest to it that is still available. If that version is El Capitan, then you're set. If it's an earlier OS, then boot into that and run the installer while you're booted in.