store result of select query into array variable

You cannot store 17 values inside a scalar variable. You can use a table variable instead.

This is how you can declare it:

DECLARE @a TABLE (id uniqueidentifier)

and how you can populate it with values from Enrollment table:

INSERT INTO @a 
SELECT EnrollmentID FROM Enrollment

You should declare @a as a Table Variable with one column of type unique identifier as follows:

DECLARE @a TABLE (uniqueId uniqueidentifier); 

INSERT INTO @a
SELECT EnrollmentID 
FROM Enrollment;

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