My Amazon Kindle App cannot be updated because my OS on my Mac needs to be updated to 11.0 and this isn't happening. Why?

iMacs whose hardware model year ( > About This Mac) predates 2014 cannot upgrade to macOS 11.0 (Big Sur). They are too old. Any Intel-based iMac released in 2014 or later can run Big Sur or better.

Since your Mac runs Catalina now, that means it must be Intel-based. As an aside: The M1 iMac originally shipped with Big Sur, although I suppose that Apple could have shipped some with Ventura or Sonoma. The just-announced M3 iMac is not able to run Big Sur; it ships with, and will require at least, Sonoma.

First check your Macs model and year via the apple logo in menu bar>about this mac

If you've checked that, please see this link to see if your Mac is compatible with macOS 11 Big Sur

macOS Big Sur is compatible with these computers - Apple Support

What is the model year of the iMac? It is very possible your iMac is too old to run versions of Mac OS later that 10.15.7. Perhaps if you read https://www.macworld.com/article/673697/what-version-of-macos-can-my-mac-run.html this will help answer the question for you.

dialabrain wrote:

Servant of Cats wrote:

iMacs whose hardware model year ( > About This Mac) predates 2014 cannot upgrade to macOS 11.0 (Big Sur).

FWIW, there was no mention of an iMac or any specific Mac.

FWIW, the OP is in the Mac > iMac community.

Servant of Cats wrote:

iMacs whose hardware model year ( > About This Mac) predates 2014 cannot upgrade to macOS 11.0 (Big Sur).

FWIW, there was no mention of an iMac or any specific Mac.

Servant of Cats wrote:

FWIW, the OP is in the Mac > iMac community.

FWIW, people often post in the wrong community.

Some people are just better mind readers.