to add to what Richard has said:
check the current sizing of your shared_pool_size, by using the following command:
SQL> show parameter shared_pool_size
10% of the value of the shared pool size will be assigned to the streams pool (also, buffer cache size will be reduced)
So, alter the value using the command:
SQL> alter system set streams_pool_size = (10%of shared pool size) scope = spfile;
and then try use expdp command again....
its better to set your to be auto-tune, in this case Oracle will maintain the memory structures.
SQL> show parameter sga_target