That message can come for many reasons, none of them of course being GSX issues:
- Your Simbrief Alias (username) is wrong, check it, double check it and triple check it. If your username was created a while ago and it contains spaces, it won't work. Simbrief now doesn't allow to create usernames with spaces in it, but if yours was created before they enforced this rule, you need to change it.
- The last dispatched flight might have expired. They expire in a few days.
- Simbrief might really be down. They did have some shortages in the past days.
- A firewall (not antivirus) is preventing GSX from reaching Simbrief. Configure it to be sure the Couatl engine has outside online access.