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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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

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

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.

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"

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

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