Your error comes from the values statement. You can not use the deleted and inserted table just like that. Those tables are like every other table in SQL server, you must use them via a select statement.

Ex.
insert AUDITTABLE (AUD_TABLENAME,AUD_COLNAME,AUD_OLD,AUD,NEW,AUD_DATE,AUD_LANID,COMPNO)
select 'customertable','cus_name',d.cus_name,i.cus_name,getdate(),i.lanid,i.compno)
from inserted i
inner join deleted d on d.cus_number = i.cus_number

Note: I assume that you have a cus_number which is the primary key of your customertable.

Another things is that you do not have to use into since your table is already created.