You should perform a Time Machine backup, and check if vendors have any replacements for 32-bit applications, printer and scanner drivers, or specific Catalina compatible versions available — before proceeding with a Catalina upgrade.
If you have any third-party software installed that has 32-bit, or Catalina incompatible kernel extensions installed, entirely remove those applications before the upgrade. Office 2008/2011, and earlier 32-bit Office 2016 suites are not supported in Catalina. Nor is any Adobe Suite older than CC, or applications that require Java 6 — which is unsupported in Catalina.
Once you have done both paragraphs, see if you can download Catalina from the Mac App Store, and proceed with the upgrade in that manner.