An Early 2015 MacBook Air can upgrade as far as Monterey, but no further. Monterey is still one of "the most recent three" major versions of macOS – for now. (That's likely to change this fall.)
32-bit applications won't work on Catalina or later (including Monterey). Check whether you have any that you rely upon, and figure out what to do about them before upgrading.
I would suggest making a bootable backup of your current system on an external drive. Then use the App Store link in How to download and install macOS - Apple Support to get and run the Monterey installer.
I am assuming that you have not installed an internal third-party SSD such as the ones that OWC sells. From what I have read in other threads, when you install Monterey, it tries to update your firmware. If your Mac does not already have the firmware update, and it has a non-Apple internal SSD, the firmware update fails, and the Monterey update fails, too. Once you've installed Monterey anywhere (even on an external drive), the firmware is in place – and you can then reportedly replace the internal Apple SSD and successfully install Monterey on an internal third-party SSD.