Fix 2: Verify Your Game Cache
This fix is for the Steam version of the game. It does occasionally happen that files will become corrupted,
Find Final Fantasy XIV in your Steam library.
Right-click and select 'Properties'.
Navigate to the 'Local Files' tab.
Select 'Verify Integrity of Game Cache'
This solution isn't the most likely to work, but it's definitely one of the easiest to try.
Fix 3: Edit Some Files
This fix is pretty easy, and works fairly well for most people. But to do it, you'll need to find your Final Fantasy folder.
Once you've found your Final Fantasy folder, you'll need to find a file named "ffxiv_boot.cfg".
Open the file with Notepad or a similar program.
Find "Boot Version Check Mode". (There should be a zero next to it)
Change the 0 to a 1.
Save the file and restart your FFXIV launcher.
people have been pointing the third one out but if its on steam u can try the second one out.