我认为您需要将img标记附加到文档中，如下所示。

$(if error)<script type="text/javascript">
var error = "$(error)";
if (error == "simultaneous session limit reached") { 
const img = document.createElement('img');
img.setAttribute('src', '/chat.gif');
img.setAttribute('alt, 'Chat Here');
document.appendChild(img);
}
else if (error == "invalid password") {
document.write("Contrasena erronea");
}
else if (error == "no valid profile found") {
document.write("Tiempo terminado, consigue otra Ficha.");
}
else if (error == "User $(username) not found") {
document.write("Mete bien el usuario.");
}
else document.write("$(error)")
</script>$(endif)