upgrading the OS on my iMac

High Sierra is the newest possible. See the following excerpt from Apple Support article for information on the last possible upgrade for a 2011 iMac: Identify your iMac model - Apple Support

iMac (21.5-inch, Mid 2011)

Model Identifier: iMac12,1

Part Number: MC309xx/A, MC812xx/A

Newest compatible operating system: macOS High Sierra

The reason it cannot be updated beyond 10.13.x is it simply does not have the hardware to update it. Before you ask, no the hardware cannot be upgraded either. If you need to run more current versions of Mac OS, then it is time to replace the 2011 with a more up-to-date Mac.

No. It can't be upgraded past High Sierra.

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