Equal\inner joins can be specified in the FROM clause or the WHERE clause. Outer joins have to be specified in the FROM clause, but you can't mix these two. So, switch the inner/equal join to using the same type of syntax:
select t1.jobnum, t1.sequence, t2.sequence
from myjobs t1 inner join myjobs t2 on t1.jobnum = t2.jobnum
left outer join myparts t4 on t1.jobnum = t4.jobnum
where t2.sequence =
(select top 1 t3.sequence from myjobs t3
where t3.jobnum = t2.jobnum and t3.completed = 0)
At this point, we don't support order by in a column or table subquery.