i have a mid 2012 power mac. my request to update my os from the app store is getting rejected. is there no way to update from high sierra to Catalina or Big Sur?

See the following from Identify your Mac Pro model - Apple Support

(and be sure to note the footnote --- * To install macOS Mojave on Mac Pro models from 2012, a Metal-capable graphics card is required.)

2012

Mac Pro (Mid 2012)

Model Identifier: MacPro5,1

Part Numbers: MD770xx/A, MD771xx/A

Newest compatible operating system: macOS Mojave*

Tech Specs: Mac Pro (Mid 2012) 

User Guide: Mac Pro (Mid 2012)

Mac Pro Server (Mid 2012)

Model Identifier: MacPro5,1

Part Number: MD772xx/A

Newest compatible operating system: macOS Mojave*

Tech Specs: Mac Pro Server (Mid 2012) 

* To install macOS Mojave on Mac Pro models from 2012, a Metal-capable graphics card is required.

I believe that what you have is not a Power Mac (PowerPC-based Macintosh), but a mid-2012 Mac Pro or Mac Pro Server.

Install macOS 10.14 Mojave on Mac Pro (Mid 2010) and Mac Pro (Mid 2012) - Apple Support

"Before you upgrade to macOS Mojave on these Mac Pro models, you need to update to macOS High Sierra 10.13.6 and then make sure that your graphics card is compatible. You also need to turn off FileVault … Don't upgrade your Mac Pro (Mid 2010) or Mac Pro (Mid 2012) to macOS Mojave directly from macOS versions prior to 10.13.6."

How to download and install macOS - Apple Support

Even with a Metal-capable card, you won’t be able to run Catalina or higher.

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