My 10.13.6 Macbook Pro won't update to Ventura. App store has no other options.

Is appears, to these old eyes, that this computer is running a Fusion Drive formatted as HFS Journaled / GUID Partition Map on a computer running macOS 10.13 High Sierra ( your wording )

Predicated on this computer is Qualifying to upGrade to Ventura

The internal Drive may needed to be converted to the APFS Drive Format which is Required for macOS 13 Ventura

Any and all credits below to the Author of below method tbirdvet

CONVERTING TO APFS - Apple Community

" It is not mandatory. If you upgrade to Catalina it will reformat to APFS automatically. If you want your internal drives to be APFS you can convert those by restarting your Mac by using Command + R to boot into recovery mode. Then, from Disk Utility>View>Show All Devices> Select your parent drive (or external) then click Edit and Convert to APFS. It will not impact your files. Before doing any conversion be sure you have a backup just in case. "

OR

https://www.ifixit.com/Guide/How+to+convert+your+Mac's+storage+from+HFS+to+APFS+file+system/150453

withan wrote:

I am being forced to update my OS from 10.13.6; however, the update repeatedly fails.

Log reads: Aug 25 08:45:25 GiuliaQrifoglio osinstallersetupd[1295]: OSISSoftwareUpdateController: Failed to load catalog https://swscan.apple.com/content/catalogs/others/index-13-12-10.16-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz: Error Domain=MSUProductManagerErrorDomain Code=51 "(null)" UserInfo={NSUnderlyingError=0x7f9ad05199c0 {Error Domain=MSUDataDownloaderErrorDomain Code=3 "(null)"}}

Aug 25 08:45:25 GiuliaQrifoglio osinstallersetupd[1295]: osinstallersetupd registered client

Aug 25 08:45:25 GiuliaQrifoglio osinstallersetupd[1295]: -[OSISTargetController isCSFusionDrive:]: failed to get LVG for disk2s3

Aug 25 08:45:25 GiuliaQrifoglio osinstallersetupd[1295]: -[OSISTargetController targetContains3TBBootcamp:]: Disk doesn't appear to have a LVUUID.

Aug 25 08:45:25 GiuliaQrifoglio osinstallersetupd[1295]: disk2s3 is not convertible to APFS: You may not install to this volume because it can not be converted to Apple File System (APFS).

Do you say what exact Mac is this...?

ref

Identify your MacBook Pro model

The older macOS installer backups on this server:

How to download and install macOS - Apple Support