Why cant I upgrade to Mac OS Mojave from Version 10.9.5 on my iMac

Why cant I upgrade to Mac OS Mohave from Version 10.9.5 on my IMac which is configured:

Model Name: iMac

  Model Identifier: iMac12,2

  Processor Name: Intel Core i5

  Processor Speed: 3.1 GHz

  Number of Processors: 1

  Total Number of Cores: 4

  L2 Cache (per Core): 256 KB

  L3 Cache: 6 MB

  Memory: 8 GB

  Boot ROM Version: IM121.0047.B21

  SMC Version (system): 1.72f2

  Serial Number (system): D2***HJW

  Hardware UUID: 2276D95A-AA6C-5636-82A6-C5A58017B23C

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