ORA-31626: job does not exist
ORA-31633: unable to create master table "MYUSER.EXPDP_MYDB_BCK"
ORA-06512: at "SYS.DBMS_SYS_ERROR", line 95
ORA-06512: at "SYS.KUPV$FT", line 871
ORA-00955: name is already used by an existing object
If you didn't write the script that runs Data Pump and have little experience with expdp you might don't know what this error means, but is pretty simple: the master table (in this case MYUSER.EXPDP_MYDB_BCK) or other object with this name exist so Data Pump cannot create it, and is needed in order to do the backup. If may exist because expdp didn't finish properly the last time it was ran so the master table was not deleted.
The solution is pretty simple too: if you don't need this table just drop it, or change the master table name in Data Pump parameters if you need this object and is not related to this Data Pump backup.
Oracle Data Pump in Oracle Database 10g