You can use multiple columns in the when clause. But you can't assign a value in the then clause as you've done:
THEN RT_UNE IS NULL
What are you trying to do? Return null if the other columns are null? If so, just use null:
set null <null>
create table t (
x int ,
y date,
z int
);
insert into t values (1, null, null);
select case when y is null and z is null then x is null else x end c
from t;
Error at Command Line : 11 Column : 49
Error report -
SQL Error: ORA-00905: missing keyword
00905. 00000 - "missing keyword"
select case when y is null and z is null then null else x end c
from t;
C
<null>