It always shows 500 internal server error when I try to install any macos from playstore. Kindly someone tell me how to update my macbook Air 2013 having os 10.10 (Since I cant get any app from app store cus they are saying I need os 10.14 or higher)

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Other may not work

Make sure there is at least 15 GB of Empty Space Available on the Drive. It is needed for the upgrade installer to download, expand and install.

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina and Big Sur Only support 64 bit and there is no workaround.

Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation.

I had same problem, please see my post https://discussions.apple.com/thread/252913639?answerId=255496827022#255496827022