对。前面的括号 on 不正确。您的查询还有其他问题。我想你想要：

create table new_table_name as
    select a.proposal_code as new_name_proposal_code,
           a.1st_client_code as new_name_1st_client_code,
           a.2nd_client_code as new_name_2nd_client_code,
           a.3rd_client_code as new_name_3rd_client_code,
           a.4th_client_code as new_name_4th_client_code,
           a.product_type as new_name_product_type,
           coalsesce(bc.2nd_client_code, bcb.2nd_client_code)
    from existing_table a left join
         existing_table bc
         on bc.2nd_client_code = a.2nd_client_code and
            a.product_type = 'clothes' left join 
         existing_table bcb
         on bcb.2nd_client_code = a.3rd_client_code and
            a.product_type in ('cars', 'bikes');

笔记：
前面没有括号 on 条款。
不 or left join . or 是布尔运算符。 left join 是集（即表和结果集）上的运算符。不要混在一起。
没有重复的表别名。
你想把这两个代码结合起来，所以你需要 coalesce() 在 select .
字符串的sql分隔符是单引号，而不是双引号。 in 比一串 or 条件。

听起来你只是想要一个复杂的标准，而不是两个连接。像这样：

proc sql;
create table new_table_name as 
select 
   a.proposal_code as new_name_proposal_code
  ,a.client_code1 as new_name_client_code1
  ,a.client_code2 as new_name_client_code2
  ,a.client_code3 as new_name_client_code3
  ,a.client_code4 as new_name_client_code4
  ,a.product_type as new_name_product_type
  ,b.client_code2 as new_name_other_client_code2
from tableA a
left join tableB b
  on (b.client_code2=a.client_code2 and a.product_type = "clothes")
  or (b.client_code2=a.client_code3 and a.product_type in ("cars","bikes"))
;
quit;

为了得到更好的答案，请张贴示例输入和所需输出。