iMac 10,1 to update High Sierra update

After update from Sierra 10.12.6 Combo, APP Store from my iMac10,1 didn't show option to update to High Sierra 10.13.

Searched [https://support.apple.com/en_US/downloads/high-sierra] but nothing before [High Sierra 10.13.6] & [macOS 10.13 Supplemental].

Seems no update download available for 10.12.6. Even [macOS High Sierra 10.13.4 Combo Update] requires macOS 10.13.0 which nowhere to be found.

Thanks.