SQL & PL/SQL :: Trigger - Send Booking ID To Audit Table

I'm trying to create a trigger that will come into play after the user has inserted a 0 for the booking evaluation, i need the booking id sent to the audit table. This is what I have tried so far:

CREATE OR REPLACE TRIGGER zero_evaluation
AFTER INSERT OR UPDATE ON booking
FOR EACH ROW
WHEN (NEW.Evaluation=0)
BEGIN
INSERT INTO audit (audit_id, booking_id, Reason)
VALUES (audit_id_seq.NEXTVAL,:NEW.booking_id, 'Contact customer for 0 evaluation');
END;

I get the following:
Line 2 PL/SQL: SQL Statement ignored
Line 3 PL/SQL: ORA-02289: sequence does not exist

This is my SQL statement:
CREATE TABLE audit
(audit_id numeric(5) not null,
booking_id numeric(5) not null,

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