You can use an APPLY for this, which allows you to access values from the outer scope.

Note that the join condition is moved to inside the subquery, because the subquery is (logically) exceuted once per row of the outer scope.

SELECT  
  se.STUDENT,
  se.COURSE,
  se.ENROLL_DT,
  st.TRACK
FROM
  [Student Enrollment] AS se
OUTER APPLY (
    SELECT TOP (1)
      st.TRACK,
      st.DECLARE_DT
    FROM 
      [Student Track] AS st
    WHERE st.DECLARE_DT <= se.ENROLL_DT
      AND st.STATUS = 'Active'
      AND st.STUDENT = se.STUDENT  -- join condition
    ORDER BY
      st.DECLARE_DT DESC
) AS st;

db<>fiddle

You can use ROW_NUMBER to join the tables.

for this to work, you should have more or equal numbers in enrolment to track

You also should not use spaces in Column or table names, it must be escaped

WITH CTE_track as (SELECT
  [STUDENT],[TRACK],  [DECLARE_DT], ROW_NUMBER() OVER(PARTITION BY STUDENT ORDER BY [DECLARE_DT]) rn
  FROM
[Student Track]),
CTE_enroll as (SELECT
  [STUDENT], [COURSE], [ENROLL_DT], ROW_NUMBER() OVER(PARTITION BY STUDENT ORDER BY [ENROLL_DT]) rn
  FROM
[Student Enrollment])
SELECT
c1.[STUDENT],c1.[COURSE],c1.[ENROLL_DT],c2.[TRACK]
FROM  CTE_enroll c1 LEFT JOIN CTE_track c2  ON c1.[STUDENT] = c2.[STUDENT] AND c1.rn = c2.rn

fiddle

You can use SQL windonw function to get the result you want in the following steps:

1. Join student_enrollment and student_track table based on student, track active status and enroll_dt should be later than declear_dt.
2. Use window function for each (student, course) to get most recent track declear_date compared to enroll_date.
3. Fetch the top row for each (student, course).

Here is the query:

WITH sorted_course AS (
SELECT
    e.student,
    e.course,
    e.enroll_dt,
    t.track,
    t.declear_dt,
    t.status,
    row_number() over (partition by e.student, e.course order by t.declear_dt desc) as row_num
FROM
    student_enrollment e 
JOIN
    student_track t
ON
    e.student = t.student
AND
    t.status = 'Active'
AND
    t.declear_dt <= e.enroll_dt
)
SELECT
    student,
    course,
    enroll_dt,
    track
FROM
    sorted_course
WHERE
    row_num = 1
ORDER BY enroll_dt