You do not need to download Maverick (10.9), Apple does not provide it unless you had previously downloaded it.
Download and install current or previous versions of the Mac operating system on compatible Mac computers.
support.apple.com
According to Apples instructions in Section 1 "Check compatibility" you need "Mavericks or later". That section also said from Mountain Lion first upgrade to El Capitan. I would recommend going from Mountain Lion to High Sierra, then to Catalina.
Apple table that shows macOS and with links to available macOS installers.
Use About This Mac to see the version of Mac operating system installed on your Mac.
support.apple.com
Here is the link to High Sierra install instructions.
Download and install current or previous versions of the Mac operating system on compatible Mac computers.
support.apple.com
Make sure you have a good backup before attempting any OS upgrades. Be aware that Catalina is only 64 bit and you will not be able to use any older 32 bit apps.