i can not get my mac to update from high sierra 10.13..6

Have a look see over here . By the way iMac 2015 can upgrade all the way to Big Sur read all points and make sure Empty Space Requirements are met for a successful upgrade. Whatever version macOS upgrading to Have a Time Machine Backup plus 2 Tested Bootable Clones each on separate HD / SSD

Welcome, Bonster70, to Apple Support Communities!

With a 2015 Mac, then the likely cause for you being unable to update, from High Sierra 10.13.6, is that you have no more updates, available, through the update panel.

Historically (with the single exception, so far, of Big Sur from Catalina) upgrades are not available through the usual update panel: Instead, you need to go through the Mac App Store, and search for upgrades, to later major macOS versions.

If you have trouble finding what you want, there, please let us know, and we can provide further assistance.

You need a 2012 or later Mac to upgrade to Mojave 10.14 and above.