Can't play online

I loaded up FH2 after doing the most recent update and went to play online and it said that I need to finish content install even though it’s telling me that its at 100% complete

Try doing a hard restart of your console:
1 - hold the button down on the Xbox One front until the unit shuts down.
2 - remove the power connection from the wall or at least the wall-side of the power pack.
3 - get coffee, help someone or twiddle your thumbs for about three minutes.
4 - reconnect the power to the external power pack.
5 - start the console as you normally would.

Then logon to Xbox Live and you can check to see if you still have a download pending, or perhaps it will have completed. If it is still pending, remove it. Following that, either you’re prompted to re-download the item or when you launch the game it will pop up.

DO NOTHING else during the download. NOTHING. Let it complete fully before attempting to play the game.