Also see following error messages:
EXP-00019: failed to process parameters, type 'EXP HELP=Y' for help

You have reached the value of license_max_sessions.

Database administrators can set a hard limit for the amount of sessions allowed to connect to a database.
This is done when you are using Named User Licenses in order to prevent violation of your license agreement

First you can check at what value to limit is set to:

SQL> show parameter license

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
license_max_sessions                 integer     30
license_max_users                    integer     20
license_sessions_warning             integer     20

Next you can get the an overview of the current sessions connected to the database:

SQL> select count(*) from v$session;

  COUNT(*)
----------
        23

When the number of connected session is equal to the license_max_sessions, you will get this error message.
In order to resolve this error you will either need to buy extra licenses and up the license_max_sessions parameter, or disconnect some idle sessions using the following command:

alter session kill session 'sid, serial#';