My MacBook is not updating

• Find out which macOS your Mac is using - Apple Support

https://support.apple.com/en-us/HT201260

"From the Apple Menu  in the corner of your screen, choose About This Mac.

You should see the macOS name, such as macOS Big Sur, followed by its

version number. If you need to know the build number as well, click the

version number to see it. Which macOS version is the latest?"

{..Likely your mac is older & may require ' Upgrade ' to a later/newest macOS..}

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

Note there are various links for most macOS download installers within article.

Much older system's hardware may balk ~ when they haven't clear path ahead.

Do you know any of the specifics of your Mac?

The model (for example: MacBook Pro 16”)

The current version of MacOS (for example: High Sierra)

What your trying to update to? (for example: Mojave)

In some cases it may be that your Mac may not support the next operating system due to its age.

Please read: Writing an effective Apple Support Commun… - Apple Community