Start with the right checks.
An unavailable update and an installation error are different problems. Keep the exact error and the macOS version you are trying to install.
What to do next.
Record the Mac model, existing macOS version and the offered update.
Check the available storage and a working internet connection.
Use Apple’s Software Update or the official Apple installer for the supported version. Avoid copies offered by unrelated download sites.
If the same error returns, use Apple’s guidance for that message and ask for help before Recovery or disk repair steps.
Check the result.
After installation, check the macOS version and open essential applications. Preserve the backup until normal work is confirmed.
Stop and get help if…
The disk reports errors, essential software is not compatible or installation asks to remove data. A major upgrade needs a plan for the applications you depend on.