How to fix this problem?

IOS

Is your device compatible to update to the new version? See the article(s) below. 

IOS 15 and iPadOS 15 Compatible Devices.  *

iOS 14 and iPadOS 14 Compatible Devices.  *

iOS 13 and iPadOS are compatible with these devices - Apple Support.  *

iOS 12 is compatible with these devices.   *

IOS 11 is compatible with these devices   *   If you can upgrade to IOS 11, you can upgrade to IOS 12

IOS 10 is compatible with these devices   -

iOS 9 Compatibility & Supported Devices List  -

OS

Check the computer requirements shown in each link to see if your computer is capable of running that OS version.

macOS Monterey is compatible with these computers.   *

macOS Big Sur is compatible with these computers

Catalina

How to upgrade to macOS Mojave - Apple Support

How to get macOS High Sierra   

Sierra download

You can try to download it at the link below. If it won't let you, then your only options are to upgrade to Sierra/High Sierra/Mojave or find an older computer where you can download it using your Apple ID.

El Capitan

Which device?

iPhone or iPad:

• Software Update.
• If your iPhone won't update.

Mac:

Click ▶︎ Software Update.

Apple Watch:

Click ▶︎ Software Update.