An easier way would have been to do what Subversion told you
svn : lancer 'svn cleanup' pour enlever les verrous
This kind of error can happen if you check out while some other transaction on your machine is in progress (for example if you run update or checkout several times by accident, or if you start a transaction and abort it, e.g. you hit Ctrl-C, or the machine crashes, whatever).
It is no problem at all, don't worry. Subversion has to create lockfiles to ensure that it doesn't modify the same files in the same working copy concurrently, and this error just tells you that there are still lockfiles for some reason. Run
svn cleanup, and all is fine.