What is the problem with your OS?
The maximum Apple supported macOS that is supported by a 2011 iMac is High Sierra (10.13.6), however you must be using Mountain Lion (10.8) or newer to upgrade to High Sierra.
Download and install current or previous versions of the Mac operating system on compatible Mac computers.
support.apple.com
If you want High Sierra then you need to do a 2 step process of upgrading to 10.8 or newer (10.8, 10.10, 10.11 or 10.12), then upgrade to 10.13.
Are you interested in an upgrade to a newer OS of 10.8, 10.10, 10.11 or 10.12? I can get links if needed.