I updated the mod for 1.20.1, the file number being 1.1.1

I loaded the game and it told me that Immersive Aircrafts only supports ad astra 1.16.6

The latest mod version of Ad Astra for 1.20.1 is 1.15.8, while the latest mod version of Ad Astra period is 1.16.5, which is only available for 1.20.4, so Ad Astra 1.16.6 doesn't exist yet

You need to allow beta versions.

I did do that, and still nothing

Ahhh typo, should be 1.15.6. Try IA 1.1.2 please.