Accidentally erased startup disk (Macbook Air 2015) while trying to erase free space

Boot your MacBook to Recovery Mode:

Use macOS Recovery on an Intel-based Mac - Apple Support

If there's no recovery partition, then boot to Internet Recovery, detailed in that same article. Then let the process install whatever version of macOS it offers. If it's earlier than what your Mac will run (macOS 12 Monterey), you can update later.

Regards.

Use Command + Option + R to attempt to access the online Big Sur installer through Internet Recovery Mode. Some Macs may only boot to the older online installer for the OS which originally shipped on the Mac from the factory regardless of the keys used for booting.

For creating a bootable macOS USB installer you will need access to another Mac model generally from 2007 to mid-2021. The exact model of this other Mac will determine which macOS USB installer you can create. This model range will allow you to create a 10.11, 10.13, 10.15, or 11.x USB installer.

You could perhaps download the Yosemite installer on a PC, but you cannot build an installation drive without a Mac. So one way or another you'll need to find someone with a Mac who can help. Perhaps there's a Mac user group in your area.

Regards.

OK, you're going to need access to a fully-working Mac. If you can get such access, you can download the Yosemite installer here:

http://updates-http.cdn-apple.com/2019/cert/061-41343-20191023-02465f92-3ab5-4c92-bfe2-b725447a070d/InstallMacOSX.dmg

or Monterey, if you want to install that version, here:

https://apps.apple.com/us/app/macos-monterey/id1576738294?mt=12

Then to create a bootable installer, follow these instructions:

Create a bootable installer for macOS - Apple Support

Once that's done, you should be able to boot your MacBook from that drive and run the installer.

As you can see, there are a number of steps involved. You may prefer to take your Mac to a third-party service center - your MacBook is considered "obsolete" now so Apple generally will not provide any service - and have them help you get things reinstalled.

Regards.

Ruiner_of_Things wrote:

Ok, thank you. I did research ways to download a macOS installer using a PC, have come across countless posts that say you can do it, only for them ultimately to link to trying to download directly from apple, which cannot be done. So is there genuinely no way to get an installer without a mac?

There is no easy method to create a bootable macOS USB installer using an unsupported system whether it is Apple or general Windows PC. Technically it can be done using lots of command line tools assuming you can find a link to the installer on Apple's servers (Apple's indexing of content on there CDN servers does not make that remotely easy to do).....assuming instructions can be found as well. With Apple constantly changing their installers, instructions that worked yesterday may not work today.

I've seen people mention various apps, but I've rarely seen any success stories. Most of the success stories came from years ago, but Apple has changed the format of their installer over the years. It is definitely frustrating that Apple cannot be bothered to provide a raw image of an installer that can be easily transferred to a USB stick using any computer or OS. I'm actually planning on writing a user tip with instructions for people to make a raw image for a bootable macOS USB installer so people can keep a copy & back it up so they have it in case of emergencies. I just haven't had time to finalize the instructions and testing to confirm the instructions work for various operating systems.

You have a pretty good range of model years so hopefully you know someone friend, family, co-worker, IT Department who may have a compatible Mac that can be used. You can use the information in the following article to confirm a particular model Mac is compatible with a version of macOS your MBAir can use since the 2007-2015 model years is just a generalization since there are always some exceptions:

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility