(USA)

Error Message: "Unable to update patch files. [11006][20815][19900][5]"
"(Drive Letter):/SquareEnix/Final Fantasy XIV - A Realm Reborn/game/ffxiv_dx11.exe"

I recently had this error when attempting to log in past my FFXIV launcher.
My solution was to update my NVIDIA GeForce Drive to the most recent one available (it would have been JAN 26, 2021 for mine) and restarted the computer. In addition I uninstalled and reinstalled the FFXIV game and launcher, and was able to rectify this error and install all of the latest files and updates. I logged in and started the game as per usual.

I hope this helps anyone having this technical issue!