Your Mac might be on the last version it can run, or your Mac might have an upgrade available, but—if Xcode is telling you it needs macOS 11 and you don’t have macOS 11—you’re not running current macOS.
Apple differentiates updates from upgrades. Updates are smaller and are not intended to be disruptive to apps, while upgrades are larger and can be and are intended to be more disruptive to apps. Checking for updates doesn’t check for and doesn’t get you upgrades.
Which Mac are you using? See the 🍎 > About This Mac menu item, and post the Mac model and year, and somebody here can tell you about the available upgrades. And whether your Mac can run macOS 11. If not, then the link to previous versions of Xcode is your path, as was mentioned in the previous reply. That, or a replacement Mac that can run current Xcode.
And for completeness, App Store submissions require current Xcode.