The error message that you are getting is sometimes caused by being idle for quite sometime inside the Raid Battle feature and some other possible reasons such as latency. To remedy or fix the issue please do try the following options or methods first;
1. Fully close Brave Frontier, re-launch then try accessing it again.
2. If the first method does not work then we suggest you to try clearing the game's user data.
3. If somehow on a rare occasion that the issue is still not resolved on your part, the next advise that we can give you is to do a Refresh Data.
Note that before performing option 2 or 3, be sure that your account is currently linked to Facebook to avoid any data loss. Check on how to connect your Facebook to account here.
If you need any guides regarding the said procedures, please read the articles below;
How to do Clear User Data
If the issue or problem do still persist, now is the time we advise you to send us your concern to us so we can directly check it from our side. If you want to know how, please do check this article.