Most likely you have the wrong user/password.
You may want to recreate your domain with a new user and password.
"AS_ADMIN_PASSWORD=<password>" is not a command. It's an entry in a file that you use to pass as an value to --passwordfile option when you start the domain.
e.g.
asadmin start-domain --passwordfie <file-name> ... <domain-name>
HTH
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