Big Sur : update not found

Hi,

I would like to upgrade my imac to Big Sur . But i keep getting "Update not found The requested version of macOS is not available."

  Model Name: iMac

  Model Identifier: iMac14,1

  Processor Name: Quad-Core Intel Core i5

  Processor Speed: 2,7 GHz

  Number of Processors: 1

  Total Number of Cores: 4

  L2 Cache (per Core): 256 KB

  L3 Cache: 4 MB

  Memory: 8 GB

  Boot ROM Version: 430.0.0.0.0

  SMC Version (system): 2.14f24

Please can you help ?