Typically this means that you have an error in your trigger. Using for example SQL*Plus run command
SHOW ERRORS TRIGGER TriggerName
To see what are the errors in comiplation.
Addition:
CREATE OR REPLACE TRIGGER "MyUser"."TRG2TEST"
BEFORE INSERT ON COMPUTER
FOR EACH ROW
BEGIN
INSERT INTO COMPUTER (COMPID,COMPNAME,CPU,hdd_size,motherboard,operatingsystem,visible,good)
VALUES
(SEQ_CompID.NextVal, :new.CompName, :new.CPU, :new.HDD_Size, :new.MotherBoard, :new.OperatingSystem,1,1);
END;
Addition 2:
Added FOR EACH ROW because this should fire separately on each new row that is inserted.