People who have experienced environmental trauma never had a sense of safety in the first place. Their earliest memories are of being in a traumatic situation. They may have been born into an unsafe environment such as a violent home or a community living under the threat of violence.
Because they have no early memory of safety to fall back, such people can be unable to feel safe even when they are in a safe environment. Lacking a sense of safety makes it difficult for them to recover from their traumas, let alone form good relationships or build a fulfilling life.
If you have experienced environmental trauma, there is a special procedure that will help you develop the ability to feel safe in safe places. Once you build your sense of safety by following this procedure, you can do the work of recovering from your single-event and sustained traumas and build a happy, fulfilling life. But you cannot do this work unless you know you are basically safe in your present environment.