Can't update OS beyond Catalina. Error message says Big Sur and so on are not available.

Part 1 of 2

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.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

Does this computer appear as Listed and qualifying for an upGrade to a newer version of macOS? 

The below links will help identify which version of macOS  this computer may Qualify to run.

Computer Qualifying for macOS 13 Ventura 

Computer Qualify for the Monterey upgrade

macOS Big Sur 11 hardware requirements

Make Time Machine Backup  is very useful 

Part 2 of 2

If the user whats the latest and greatest versions of macOS, all is not lost.

Explore the Apple Trade-In  Process where the Value of the exiting device could be used against the cost of a new Apple Computer, iPhone or iPad  

Or Shop Refurbished Apple computers at an Apple Reduced Cost

Have used both previously and have been satisfied each time.

See macOS Big Sur is compatible with these computers – Apple Support (AU)

If your iMac is too old then it probably has the latest version.

Find out which iMac you have and then whether the next version of the OS supports it.