You need to use conditional aggregation.

Untested without any consumable sample data, however something like this:

select Status, Count(*) as amount
from table_jobs
cross apply(
  values(Iif(assignment_status in ('to_fix', 'to_validate'), 'Pending', assignment_status))
)s(Status)
group by Status;

Stu's answer is probably the right one. Here are a few alternative ways to do it:

-- Write conditional expression directly in group by and select list
-- Downside, duplicates that big expression. Upside, succinct? maybe?
select
    Assignment_status = iif(assignment_status in ('to_fix', 'to_validate'), 'pending', assignment_status),
    amount = count(1)
from table_jobs
group by iif(assignment_status in ('to_fix', 'to_validate'), 'pending', assignment_status)

-- Write conditional expression as a subquery
-- Downside, less succint, upside, only writes the expression once
select
    assignment_status,
    amount = count(1)
from
(
    select
        Assignment_status = iif(assignment_status in ('to_fix', 'to_validate'), 'pending', assignment_status),
        *
    select table_jobs
) a
group by assignment_status