The Age of this computer will Govern what newer version of macOS it will Qualify to install.
Use About this Mac from the Apple Icon on Desktop - the required Information will be there.
With the above information in-hand - follow this ➡️ How to get old versions of macOS ⬅️ and verify what version this computer Qualifies to install. For Best results use Safari to commence the download as Others may not work.
Safari will open a redirect and open the Apple Store where the download can commence.
In your computer case Really suggest updating to Current version of Monterey 12.7 Before attempting the Ventura 13.6
Keep your Mac up to date
About the security content of macOS Monterey 12.7
If Yes, 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.
The below links will help identify which version of macOS this computer may Qualify to run.
Computer Qualifying for macOS 13 Ventura
Computer Qualify for the Monterey upgrade
Make Time Machine Backup before doing any upGrades