Database Tuning Advisor CLI "dta" missing
DTA used to be a part of SQL Server, but it moved to SSME.  During the upgrade from SQL Server 2014 to SQL server 2017, two things happened:

SQL Server 2014 DTA.EXE was left behind
SSMS 18 did not add the directory containing DTA.EXE to the path

Therefore, dta.exe was brining up the SQL Server 2014 DTA, which no longer had license information and was failing.
The fix is to add the SSMS directory containing DTA.EXE to the path, before any other SQL Server directories.  See https://stackoverflow.com/q/44272416/238886 for how to edit the path in Windows 10.  Here's the directory to add.  Make sure it's above the other SQL Server directories, as shown here: