No action is required on your part. Your Brave Rewards profile will be automatically re-evaluated on a regular basis to determine when (or if) the flag can be removed.
Just as that notice says, no action is required. The system will routinely check and if your account can be unflagged, it will do so. Brave will not advice why your account was flagged, as they don’t want to let people know the types of activity the system is picking up on. The reason for that is they want to give as little help to people abusing and scamming the system as possible.