Trying to reinstall MacOS Catalina on MacBook Air. Wants me to reformat to apfs. Help?

Since this is a fairly new notebook, why did you want to reinstall macOS Catalina? Regardless, is it currently running successfully? ... or did you post here because you are having an issue with it and decided to reinstall macOS? I would have to assume the latter as that is what you provided in your initial post.

... and if that is correct, then the following two support articles are the ones you need to familiarize yourself with to do so:

(Note: I'm assuming that this is an Intel-based CPU Mac and not an M1 model.)

• Use Disk Utility to erase an Intel-based Mac - Apple Support
• How to reinstall macOS - Apple Support

MacBook Air A2179 models came preinstalled with Catalina (macOS 10.15) and a 512GB SSD system drive (unless you had upgraded to a larger SSD when you purchased the MBAir). Catalina requires APFS and your system drive had to have already been in APFS format when you purchased the MBAir. Uncertain why you want to reinstall at this point unless you had a problem.

Since it was preinstalled with Catalina, if you boot into Recovery Mode you should automatically be able to reinstall Catalina.

To boot into Recovery Mode use either of these methods:

Command (⌘)-R:  - Will start up from the built-in macOS Recovery system. This should give you a Catalina reinstall.

Option-Command(⌘)-R or Shift-Option-Command-R - Will start up from macOS Recovery over the internet. This may or may not give you Catalina as an option; more likely it will give you macOS Monterey.

I just want to be sure I understand. In order for me to be able to, I need to ask you a few questions. I'm sorry if I am being redundant on some of them, but they are important.

1. Which exact version of MacBook Air do you have? There are both Intel & M1 versions that came out in 2020.
2. What version of macOS is it running now?
3. Can it successfully boot up to the Mac desktop?

With these question answered, I can assist you with getting it back to whatever state your goals for it are.

macOS Big Sur would be the next newest operating system after Catalina. You can elect to upgrade to it or stay with Catalina. It is really your call. You don't mention which exact version your MacBook Air is, and it may be possible that it can be upgraded all the way up to the (currently) latest version: macOS Monterey. Again, there is no hard requirement for you to do so at this time.

If you notebook is meeting your computing requirements running Catalina, then I suggest that you stay with it at this time. However, if there is some issue that you needed to reinstall Catalina, we would need a bit more information from you to assist you with this.

Thank you. Ok, it's an Intel-based Mac.

The two support articles I provided you earlier should take you through the steps. The first one covers how to use the Disk Utility in Recovery Mode to completely erase the system drive. Be sure to read each step carefully before performing them.

Once successfully erased, your Mac will still be in Recovery Mode. The second article takes you through the steps to reinstall macOS while in Recovery Mode. Again, read through it carefully.

If either of these documents seem too difficult to follow, I would strongly suggest that you contact Apple Support directly or pay a visit to your local Apple Store to have them lend you a hand. If you prefer the former, you can simply use the "Get Support" link in the upper right-hand side of this forum's web page to get started.

Great! If you get stuck or have any additional questions, please don't hesitate to let us know. Before you know it, you'll be helping others with how to do this.

... and Merry Christmas to you as well.

The reinstallation of macOS Catalina, requesting to reformat the boot drive to APFS, would be typical ... especially if the drive is an SSD. What exactly do you need help with?