Hi friends! I've found the solution!
Jean, I know the log was saying otherwise, I'm not a computer newbie, and I had assured the sim was NOT running as admin, or in any compatibility fix mode. Turned out it was not the problem.
What happened is I had copied in that folder (C:\ProgramData\Garmin\Trainers\Databases\FPLN) a personal readme.txt file (on how to create a fltplan) and THAT WAS THE PROBLEM! By removing it, the import function was back. So I presume we cannot add anything else
we cannot add anything else in that folder, except flight plans and the user file.
I have discovered the reason why I did not have import functionality when using the RealityXP GTN 750. The Simbrief downloader application does not include a RealityXP .gdp export option so I used the F1 .gdp export. The Simbrief downloader places a dash between the airport ID's (ex. KMOB-KNEW) in the .gdp file name which is not recognized by the RealityXP unit.
Once I replaced the dash with an underscore (ex. KMOB_KNEW) I regained use of the GTN 750 import functionality and all my Simbrief
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