When clicking on the client launcher, the first thing you will see is the patcher. This is when the game checks to see if your version of the game is up to date and if not, updates automatically.
A "Full Check" will check all the game's files on your computer for any possible corruption or conflicts.
You can start the Full Check by clicking the "Full Check" button on the patcher.
The full check will take several minutes to complete. Please do not close the patcher during this time. Once finished, if no other messages appear, then that should mean that the client is ok to run. If there are conflicts with the game files, a message should appear. If this happens, please screenshot/copy the message and submit it to the Support Team via ticket