1. You can't go there alone.
2. You are not able to go there alone.
3. You are not allowed to go there alone.
(What does #1 mean? Does #1 mean #2 or #3?)
1 and 2 mean the same thing. The words "can" and "able to" mean that one is physically capable of performing the action, in this case going (walking? riding? climbing? or ?) somewhere.
In 3, "allowed" means you have permission to go somewhere.