What is the latest OS to which I can upgrade an early 2015 MacBook Air running Sierra (10.12)?
Apple states you can upgrade to Big Sur directly from Sierra. You say you can not find Big Sur in the Apps store. Perhaps, you can use a link from this Apple website.
The recommendation to first upgrade to High Sierra not uncommon. High Sierra includes a firmware upgrade that allows booting from Apple's new APFS format. Although if you upgrade directly to Big Sur, the firmware upgrade will still take place. Apple offers this webpage for information on how to obtain and install High Sierra.
You should be aware that do to a the firmware upgrade, installation can take longer that you would normally expect. If your computer seems stop working during the upgrade, be patient this may be normal.