How to: Determine which .NET Framework versions are installed (Microsoft)
Current Duplicati needs 4.7.1. Minimum version says Release >= 461308 in
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full
which can be seen in Registry Editor or probably this edit of their PowerShell:
(Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full").Release -ge 461308
I’m more familiar with non-server, which seem to do well at updating versions.
Install the .NET Framework on Windows 10 and Windows Server 2016 and later
at its top is possibly the exact message you saw wanting 4.7.1. Did install fail?
is very vague, but I don’t know if we’re well-equipped to troubleshoot Windows.
More specifically you need 4.7.1 but might as well run on 4.8 or whatever’s new,
however there was big change at .NET 5 and beyond. Ignore those for Duplicati.