Corrupted AdskLicensing service. Fix: Open Command Prompt as Admin → cd C:\Program Files (x86)\Common Files\Autodesk Shared\AdskLicensing\Current\helper → AdskLicensingInstaller.exe --uninstall → Reinstall licensing from Autodesk's website.
Setup.exe /W /Q /I deployment_name.ini /language en-US
AVA provides direct HTTP links to all old versions. Search "Autodesk Virtual Agent" → Request Revit 2019 (both 32-bit and 64-bit links available).