Unable to upgrade my Macbook Air early 2015 Operating system upgrade

Hard to say, while you should be able to upgrade the macOS to a much later version;

that also should include a newer matching iMovie (if it remained a stock app in OS.)

A new(er) macOS is supplied by the official downloadable installer; these are kinda

large and take time to download and build a new system after they/themselves are

installed. Sometimes they will stall or not appear to progress, for several hours.

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

This would also upgrade the native Safari browser; as far as that goes.

These browsers are tied to macOS upgrade/updates; for awhile they

are current. Then later, they fade. (You would still need to use Safari

to get & use these macOS 'installer downloads' from Apple Servers.)

• Go64 - "A fast, free way to get a comprehensive look at your 64-bit

app compatibility & avoid surprises before installing macOS Catalina."

https://www.stclairsoft.com/Go64/

Other issues may result if you do not know the effects of upgrading to Mojave 10.14.6

or later; after Mojave macOS. Once past (in Catalina), there's no support for older 32-bit

software drivers, for Catalina. Certain software names may not have a newer version

if the maker didn't build one. This is also a consideration, in an upgrade to later OS.

Also, suspect the Big Sur macOS11.x (may (or not) be a comfortable path to take.