MacOS 10.15 is not offered as an update for my MacBook Pro 2017.

That's a bit of nonsense, the macOS Catalina and Big Sur are both upgrades:

Available as 'downloadable installers' from Apple servers. (That takes time.)

(Note: 32-bit software drivers which may be in your older Mac would need

to be upgraded to their 64-bit counterparts for use with newer than Mojave.)

hardware requirements:

• macOS Big Sur 11 hardware requirements
• macOS Catalina 10.15 hardware requirements

system downloadable installers:*

• macOS Big Sur 11 can upgrade Catalina, Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks
• macOS Catalina 10.15 can upgrade Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks

These are to be found among others 'not offered' from the main repository online:

• How to get old versions of macOS - Apple Support

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

Read important details on how to download these; using Safari,

and other general information about them. There is more..

Should you need to make a bootable installer, requires

a second Mac of certain specs to make &/or use.

• How to create a bootable installer for macOS - Apple Support

https://support.apple.com/en-us/HT201372

*If you can get one of these later macOS install downloads

to appear in your Mac, that would appear in Applications

Folder. It may balk, or not; you may choose to see if Safe

mode on startup could help an installation. Might not be

needed; sometimes a simple restart helps anytime. [Just as

a bootable installer is not necessary.]

Be sure to have good backups on external USB (Time Machine).

• How to use safe mode on your Mac - Apple Support

https://support.apple.com/en-us/HT201262

Good luck & happy trails! ☺︎ ❄ ⚖︎ 🂡

Gracie_Asks Said:

"MacOS 10.15 is not offered as an update for my MacBook Pro 2017.: Hi! I need to update Lightroom, which is only compatible with MacOS 10.15 or later. My MacBook Pro 2017 (Two Thunderbolt, Three Port) has updated to its highest potential of updates the computer offers: MacOS 10.13.6.[...]"

-------

Three Things:

A. Single-Out this User:

Create a new administrator, seeing if you can install from it. If so, then it is an issue with the current user (perhaps a misconfiguration?). If not, then it may be an issue with the entire macOS, in which case you'd resin tall the Mac OS, as mentioned below.

B. Try Reinstalling the macOS:

Being an intermittent issues, start by reinstalling the MacOS. Nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be. Go Here: How to reinstall macOS from macOS Recovery - Apple Support

C. Important:

Create a Time Machine Backup, First:

Prior to the re-installation of your macOS, be certain to backup your Mac using a Time Machine Backup. That way, you can have something to restore your Mac from, should anything go wrong with the reinstall. Use an external hard drive that is at least 1.5 the total storage capacity.

Is your Mac on this list:

https://support.apple.com/en-us/HT212551

if so… then you can Update to the latest OS

PS:

A precautionary tale -- do not upgrade 'beyond known' limits.

The "Latest" has been causing issues with certain older Macs.

These may not be evident, unless you read the problems as

those upgrades generate new issues. ~Monterery 12.x could

be said to be a "bridge too far." Hardware bits, that macOS12

needs, may be lacking; or will not be fully supported.

(When all that Mac needs is a newer macOS, not last or latest;

for Lightroom. ~The resource allocation, may not support this.)

Big Sur?

That is recent enough to have most issues fixed; it had big problems. 🤖☃️