After fsx is activated it won't play except in demo mode even though the install says it is activated and I get an error message  saying it was not activated after the demo mode ends.  The install says it was successfully activated and then says it wasn't activated?

They may have turned off the activation server.

You may like to try it on Steam.  This  has been updated by Dovetail and is a real improvement though nothing stops using the old add ons.  You can get addons from steam but all your own  can be used just intall them in it on your comp.  

Costs about $6 so not too much to lose.  Strongly recomended.

Make sure you buy this not msfs2020

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Microsoft Flight Simulator X: Steam Edition

The activation server is not "turned off", despite the many rumours and suggestions that it has been.
If this is FSX Gold, did you activate both FSX and Acceleration?

1 hour ago, harrry said:

They may have turned off the activation server.

They didn`t do that.

The solution that worked for me is to edit this registry entry:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4d36e967-e325-11ce-bfc1-08002be10318}     

change small caps partmgr to CAPS PartMagr

 

ref here: https://answers.microsoft.com/en-us/windows/forum/all/fsx-activation/18f51dcc-f6c7-465f-a5a6-4ec34e5c9db2

great.

I had so much messing round trying to install my fsx that I changed to the steam one and the changes are remarkable.  

 

When I switched to this new computer, I made the switch to FSX: Steam Edition as well.  Glad I did as the Steam Edition is more stable and generally runs better than the boxed version of FSX.