I always encounter error when upgrading my macOS Big sur

rolandotorresjr wrote:

After downloading completely the 12.18 GB, an error message appears.

That could be the problem. The installer should be 12.6 GB. It seems it didn't download completely. Try deleting "Install macOS Big Sur" from the Applications folder and download it again.

What is the error message?

That's not very helpful. :)

You need at least 35.5GBs of free space on the drive, if that helps.