help- not able to update beyond macOS Sierra 10.12.6

[kaartman]

All, I need to updated my iMac os to 10.13.
Right now it has Sierra 10.12.6 but, it is not giving me an option to update OS (I checked update OS is enabled is app store.
Am I missing anything?
Below are the specs of my iMac
iMac (21.5-inch, Late 2015)
2.8 GHz Intel Core i5
8 GB 1867 MHz DDR3

Thanks.

 

[CoastalOR]

See Step 4 for the High Sierra App Store link:

How to download and install macOS - Apple Support

Download and install current or previous versions of the Mac operating system on compatible Mac computers.

support.apple.com