http://eu.finalfantasyxiv.com/lodest...1e2bc04d014cbf

A feature has been added for disabling the display of battle-related error messages that appear at the top of the screen.
To turn off these messages, disable the “Display error messages when actions fail” option under Character Configuration -> Log Window Settings.
* Messages affected by the “Display recast timer error messages” setting are also affected by this new setting.

The complete list of messages affected by this setting is as follows:
Not yet ready.
Target is not in range.
Cannot use yet.
Cannot use while casting.
Not enough HP.
Not enough MP.
Not enough TP

Read the notes.