If you are using the datapump (expdp or impdp etc.) and you get the following errors:

ORA-39002: invalid operation
ORA-39070: Unable to open the log file.
ORA-39087: directory name DATA_PUMP_DIR is invalid

There is no directory defined. You have to define it on the server.
Choose a user with the rights to create an directory, start sqlplus and enter following

create directory data_pump_dir as '\exphd\datapump';

for windows...

create directory data_pump_dir as 'x:\exportdrive\datapump';

Be sure, that the directory exists on the os before you define it in the database!