It looks like you copied the wrong code. This is what you want to add to </head>
tab:
<script type="text/discourse-plugin" version="0.8.18">
api.onPageChange(() => {
if(api.getCurrentUser() != null) {
$("#logged-in-user").text(" " + api.getCurrentUser().name.split(' ')[0]);
}
});
</script>
And place this in the After Header tab where appropriate:
<h2 class="x-title">Hey<span id="logged-in-user"></span>! Bienvenue dans ta nouvelle communauté</h2>