Before adding the aaa authorization command, use "test aaa authorization" command to ensure that there is proper connectivity to the tacacs server and correct permissions are provided to the user.
Refer following link:
http://www.cisco.com/c/en/us/td/docs/security/asa/asa-command-reference/T-Z/cmdref4/t1.html#pgfId-1562072
If this is failing then check the aaa authentication configuration:
http://www.cisco.com/c/en/us/td/docs/security/asa/asa90/configuration/guide/asa_90_cli_config/aaa_servers.html
Hope it helps.
R.Seth