Upgrading to High Sierra or newer will automatically cause conversation to APFS when installing to a SSD. My understanding is you probably want to upgrade to High Sierra before upgrading further. This generally though to be because High Sierra installs the firmware upgrade to allow booting from APFS.
Possible steps to upgrade.
- Using another newer computer, download El Capitan from the Apple website How to download macOS. Transfer the file to your Mac. Use this file to create the Install OS X El Capitan application.
- Either run the Install OS X El Capitan application to upgrade to El Capitan, or create a bootable USB installer and use that to install El Capitan.
- Boot to El Capitan and install all the updates. This will give you access to the Apps Store.
- From your Mac, go to the Apple website How to download macOS and follow the instructions to download other installer applications. I would recommend installing High Sierra first before Catalina. Note: Some installer applications may not be full installers. This means you can run the application to upgrade, but you may not be able to create a bootable USB installer.
How to Download El Capitan from Mavericks
If you do not have another computer to download El Capitan, then you may need to download using Mavericks. Steps are below.
Open a new document in the TextEdit and enter the following URL. This URL came from the El Capitan link given in the Apple website How to download macOS.
http://updates-http.cdn-apple.com/2019/cert/061-41424-20191024-218af9ec-cf50-4516-9011-228c78eda3d2/InstallMacOSX.dmg
Copy and paste the URL into Safari. The file should download.