Converting SQL Server script to PostgreSQL - sql-server

Question

There is this script in the SQL Server that needs to be converted in PostgreSQL.
Here is the script:
UPDATE Categories_convert SET Categories_convert.ParentID =
Categories_convert_1.CategoryID
FROM Categories_convert LEFT OUTER JOIN Categories_convert AS
Categories_convert_1 ON Categories_convert.MainGroupID_FK =
Categories_convert_1.MainGroupID
WHERE (((Categories_convert.Level)=2));
Then I tried to convert it to postgres. Here is the script:
UPDATE x_tmp_categories_convert SET orig.parentid =
cat2.categoryid
FROM x_tmp_categories_convert LEFT OUTER JOIN x_tmp_categories_convert AS
cat2 ON x_tmp_categories_convert.maingroupid_fk =
x_tmp_categories_convert.maingroupid
WHERE (((cat.level)=2));
Note that I have already created the table Categories_convert of SQLServer to Postgresql and renamed it to x_tmp_categories_convert .
All the fields in postgresql is in lowercase.
Now the problem is when i execute the converted script to postgresql, an error will occur:
ERROR: table name "x_tmp_categories_convert" specified more than once
SQL state: 42712
What I do wrong in the conversion?
UPDATE:
I have tried #a_horse_with_no_name 's answer but it didn't update the records at all. The parentid field is still empty. It is supposed to map all the parentid of that categor based on its maingoupid_fk.
Below is a snapshot of the records after executing the suggested script.
I have opted out the name for disclosure reasons.
Records snapshot link
UPDATE v2:
I am using php to migrate the data so pardon me for the variables used.
Here are the 2 insert statements used before the questioned update script is executed:
INSERT INTO x_tmp_categories_convert(maingroupid,name,vendorid,level,parentid)
VALUES ($id,'$mainGroup',$vendorID,1,Null);
INSERT INTO x_tmp_categories_convert(subgroupid,maingroupid_fk,name,vendorid,level)
VALUES ($id,'$mainGroupId','$subGroup',$vendorID,2);
Also, this is the table definition of the x_tmp_categories_convert table:
CREATE TABLE x_tmp_categories_convert
(
categoryid serial NOT NULL,
parentid double precision,
name character varying(255),
level double precision,
vendorid double precision,
maingroupid integer,
subgroupid integer,
maingroupid_fk integer,
pageid integer,
subgroupid_fk integer,
CONSTRAINT code_pk PRIMARY KEY (categoryid)
)
WITH (
OIDS=FALSE
);
UPDATE v3:
Already SOLVED by a_horse_with_no_name. Thank you

Answer 1

I have edited #a_horse_with_no_name 's answer and to make it work. I guess it is just a copy paste error.
Here is the final script:
UPDATE x_tmp_categories_convert
SET parentid = cat.categoryid
FROM x_tmp_categories_convert AS cat
WHERE x_tmp_categories_convert.maingroupid_fk = cat.maingroupid
AND x_tmp_categories_convert.level = 2;
For #a_horse_with_no_name, I am very grateful to you as I cannot come up with the solution without your help. +1 for you man. Thank you