Can't update OS

The latest is not an Update, it's an UPGRADE. You treat it like a new purchase, on sale for $0.

Be certain you have a Trusted disk-based Backup before you begin. The latest upgrade re-arranges the disk quite a bit.

How to upgrade to macOS Big Sur - Apple Support

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Try: https://apps.apple.com/us/app/macos-mojave/id1398502828?ls=1&mt=12

macOS versions will be available using the App Store app, not by using Software Update.

Within the last two weeks all older OS system download info was moved to a new article that combines all the available download links. Please bookmark this link for future reference:

How to get old versions of macOS – Apple Support

cfrombethesda wrote:

I have the same issue. Do you consider Samsung Potable SSD a trusted backup disk system?

It's not what I consider Trusted.

If your drive blows up during the upgrade, you lose EVERYTHING. Do YOU consider this disk-based backup one that YOU trust? if not, make another backup before proceeding.

I have the same issue. Do you consider Samsung Potable SSD a trusted backup disk system?

My MacBook Pro (15 inch) 2018, V. 10.15.7. Memory 32 GB 2400 MHz DDR4.

Thanks

Some MacBook Pro sub-models cannot be upgraded to newer OS version, and you have not told us your MacBook Pro's sub-model. From "About this Mac"