Recently I was trying to address and issue with a corrupted arbitration mailbox. The quick fix for this issue is to delete these mailboxes and the system accounts used by them and run “setup.com /PrepareAD” to recreate them.
When I tried to run: setup /PrepareAD /IAcceptExchangeServerLicenseTerms I was getting this error:
Exchange Server 2016 requires .NET Framework 4.5 or later. Download it from http://go.microsoft.com/fwlink/?LinkId=260991.
I carried out several steps to make sure .NET was installed and working correctly. The system had 4.5.2, 4.6, 4.6.1, & 4.6.2 on it. But Exchange was running 2016 CU3 on Windows 2012 R2, which only supports 4.6.1.
FIX: For the.NET 4.5 error was to upgrade Exchange 2016 to CU4. After upgrading the servers, the SETUP command now runs fine.
.NET 4.6.1 was not supported on Exchange 2013/2016 until CU13/2, article on the lack of support initially. The June 2016 updates officially added support for 4.6.1. The September 2016 updates, CU14/3, did add support for 4.6.2, but ONLY on Windows 2016. The December 2016 updates, CU 15/4, finally add support for 4.6.2 on Windows 2012 R2.
Jason (Izzy) Sherry's Blog 
The setup.exe for CU8 still says it needs 4.5 or later when the page below says 4.6.2 or 4.7.1
https://technet.microsoft.com/en-us/library/ff728623(v=exchg.150).aspx
I have wasted loads of time with this, so hope it helps someone else!
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I’ve have CU11 installed and it’s suddenly started doing it to me.
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