MacBook Pro 2018 not compatible with MacOs Sonoma

A - Suggest having at least 50 GB of Empty Space on the drive.

It will allow the Full Version ( about 12.xx GB ) to download, expand and then commence to install.

If there is Third Party Security Software installed - remove it.

It will interfere with both the download and installation process.

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Safe Mode will also eliminate Third Party Software, extensions and drivers from loading. It will only load the Minimum amount of Core Apple Processes to allow the the computer to function at a reduced Level of Performance 

3 - Try downloading Sonoma again while still in Safe Mode

First, a 15" 2018 MacBook Pro is compatible with Sonoma, a 15" 2017 MacBook Pro is not. Assuming you have the former, there is too little information supplied to help. It could be incompatible third-party software you have installed or something else.

Install EtreCheck and post the report. Allow Full Disk Access.

https://etrecheck.com/

Use the Additional Text icon to paste in the report.