Invalid Column Name on CONCAT_WS
You can use only columns from the joined tables or in your case concatenated  version of it
SELECT
    first_name, last_name, address_1_zip,
    CONCAT_WS(',', last_name,first_name) AS CombinedName
FROM
    CPSQL.dbo.legal_entity AS LE
INNER JOIN CPSQL.dbo.cases as C ON CONCAT_WS(', ', LE.last_name,LE.first_name)=C.style
WHERE C.create_date &gt;= Convert(datetime, '2021-03-01') AND C.create_date &lt;= Convert(datetime, '2021-04-16')
ORDER BY C.create_date



CREATE TABLE legal_entity
    ([first_name] varchar(7), [last_name] varchar(7), [address_1_zip] int)
;
    
INSERT INTO legal_entity
    ([first_name], [last_name], [address_1_zip])
VALUES
    ('Maria', 'Munoz', 33122),
    ('John', 'Glenn', 10003),
    ('Melissa', 'Johnson', 90210)
;




CREATE TABLE cases
    ([style] varchar(22), [case_number] varchar(7), [create_date] varchar(23))
;
    
INSERT INTO cases
    ([style], [case_number], [create_date])
VALUES
    ('Munoz, Maria', 'SL12345', '2021-03-01 11:45:26.657'),
    ('Glenn, John', 'SL45678', '2021-03-02 12:10:14.552'),
    ('Johnson, Melissa (DNY)', 'SG42561', '2021-04-10 01:06:10.214')
;




SELECT
    first_name, last_name, address_1_zip,
    CONCAT_WS(',', last_name,first_name) AS CombinedName
FROM
    legal_entity AS LE
INNER JOIN cases as C ON CONCAT_WS(', ', LE.last_name,LE.first_name)=C.style
WHERE C.create_date &gt;= Convert(datetime, '2021-03-01') AND C.create_date &lt;= Convert(datetime, '2021-04-16')
ORDER BY C.create_date
GO


first_name | last_name | address_1_zip | CombinedName
:--------- | :-------- | ------------: | :-----------
Maria      | Munoz     |         33122 | Munoz,Maria 
John       | Glenn     |         10003 | Glenn,John  


db&lt;&gt;fiddle here


CREATE TABLE legal_entity
    ([first_name] varchar(7), [last_name] varchar(7), [address_1_zip] int)
;
    
INSERT INTO legal_entity
    ([first_name], [last_name], [address_1_zip])
VALUES
    ('Maria', 'Munoz', 33122),
    ('John', 'Glenn', 10003),
    ('Melissa', 'Johnson', 90210)
;
GO




CREATE TABLE cases
    ([style] varchar(22), [case_number] varchar(7), [create_date] varchar(23))
;
    
INSERT INTO cases
    ([style], [case_number], [create_date])
VALUES
    ('Munoz, Maria', 'SL12345', '2021-03-01 11:45:26.657'),
    ('Glenn, John', 'SL45678', '2021-03-02 12:10:14.552'),
    ('Johnson, Melissa (DNY)', 'SG42561', '2021-04-10 01:06:10.214')
;
GO




SELECT
    first_name, last_name, address_1_zip,
    CONCAT_WS(',', last_name,first_name) AS CombinedName
FROM
    legal_entity AS LE
INNER JOIN cases as C ON PATINDEX( CONCAT( LE.last_name,', ',LE.first_name,'%'),C.style) &gt; 0
WHERE C.create_date &gt;= Convert(datetime, '2021-03-01') AND C.create_date &lt;= Convert(datetime, '2021-04-16')
ORDER BY C.create_date
GO


first_name | last_name | address_1_zip | CombinedName   
:--------- | :-------- | ------------: | :--------------
Maria      | Munoz     |         33122 | Munoz,Maria    
John       | Glenn     |         10003 | Glenn,John     
Melissa    | Johnson   |         90210 | Johnson,Melissa


db&lt;&gt;fiddle here