Receiving error "invalid identifier" while trying to update a table in Oracle
Your update query doesn't seem complete or it would give another error message , also it helps to make a [mre].


create table TEST_FILE_MERGE
(
  customer_num    NUMBER,
  CUST_FIRST_NAME VARCHAR2(180),
  CUST_LAST_NAME  VARCHAR2(180),
  r_id            NVARCHAR2(1000)
  ,
  CONSTRAINT cumstom_unique UNIQUE (customer_num)
);




INSERT INTO TEST_FILE_MERGE VALUES( 123,'perter','foe','1')




create table REQUEST
(
  req_id              NVARCHAR2(1000),
  cust_no             VARCHAR2(50),
  customer_first_name VARCHAR2(50),
  customer_last_name  VARCHAR2(50),
  error_code          NUMBER,
  &quot;IS_CHECKED&quot;          NUMBER
);




INSERT INTO REQUEST VALUES (1,123,'Peter','Foe',0,0)




UPDATE (SELECT A.CUST_FIRST_NAME, A.CUST_LAST_NAME
,B.CUSTOMER_FIRST_NAME,B.CUSTOMER_LAST_NAME,B.ERROR_CODE,B.IS_CHECKED  
          FROM TEST_FILE_MERGE A
         INNER JOIN REQUEST B
            ON A.CUSTOMER_NUM = B.CUST_NO)
   SET CUSTOMER_FIRST_NAME  = CUST_FIRST_NAME,
       CUSTOMER_LAST_NAME   = CUST_LAST_NAME,
       ERROR_CODE           = 0,
       IS_CHECKED           = 1;


1 rows affected




SELECT * FROM REQUEST


REQ_ID | CUST_NO | CUSTOMER_FIRST_NAME | CUSTOMER_LAST_NAME | ERROR_CODE | IS_CHECKED
:----- | :------ | :------------------ | :----------------- | ---------: | ---------:
1      | 123     | perter              | foe                |          0 |          1


db&lt;&gt;fiddle here
db&lt;&gt;fiddle here