How do I change a table with 2 columns where I have repeating values in both columns into a matrix?

RMF40 If you have Excel for Microsoft 365, you could try something along these lines (using the new PIVOTBY function):

=LET(
    a, FILTER(A2:B1000,A2:A1000<>""),
    r, CHOOSECOLS(a,1),
    v, CHOOSECOLS(a,2),
    n, ROWS(r),
    c, "Subject "&CHAR(MMULT((SEQUENCE(n)>=SEQUENCE(,n))*(r=TOROW(r)),EXPAND(1,n,,1))+64),
    PIVOTBY(r,c,v,CONCAT,0,0,,0)
)

See attached...

from my perspective, I guess a pivot table will doRMF40 

RMF40 

As variant that could be Power Query

let
    Source = Excel.CurrentWorkbook(){[Name="Students"]}[Content],
    GroupNames = Table.Group(
        Source, {"Name"},
        {
            {"Subject", each List.Sort( [Subject] )}
        }),
    SubjectNames = List.Transform(
        {1..List.Max( Table.TransformColumns(
            GroupNames,
            {"Subject", (q) => List.Count(q)} )[Subject] )},
            (e) => "Subject " & Character.FromNumber(e+64)
        ),
    ExtractSubjects = Table.TransformColumns(
        GroupNames,
        {"Subject", each Text.Combine(List.Transform(_, Text.From), ","), type text}
    ),
    SplitSubjects = Table.SplitColumn(
        ExtractSubjects,
        "Subject",
        Splitter.SplitTextByDelimiter(",", QuoteStyle.Csv), SubjectNames)
in
    SplitSubjects

Please check attached.

RMF40 

A Power Query variant attached:

let
    Source = TableSource,
    GroupedName = Table.Group(Source, {"Name"},
        {
            {"DATA", each
                Table.Transpose(
                    Table.SelectColumns(_, {"Subject"})
                ),
                type table
            },
            {"Count", Table.RowCount, Int64.Type}
        }
    ),
    RemovedCount = Table.RemoveColumns(GroupedName, {"Count"}),
    SortedName = Table.Sort(RemovedCount, {{"Name", Order.Ascending}}),
    ListMaxCount = { 1..List.Max( GroupedName[Count] ) },
    ExpandedSubject = Table.ExpandTableColumn(SortedName, "DATA",
        List.Transform( ListMaxCount, each "Column" & Text.From(_) ),
        List.Transform( ListMaxCount, each "Subject " & Character.FromNumber(_ +64) )
    )
in
    ExpandedSubject

RMF40 

Attached, with Power Query then Power Pivot

RMF40 

All you are trying to do is generated a list of subjects for each student.  Unfortunately the current implementation of Excel will simply return an error message "Nested arrays are not supported".  Not very helpful when the whole purpose of the calculation is to return a nested array"!  A possible solution is to use

=LET(
    student,      UNIQUE(Name),
    SubjectListλ, LAMBDA(n, TOROW(FILTER(Subject, Name = n))),
    subjectList,  MAPλ(student, SubjectListλ),
    SORT(HSTACK(student, subjectList))
)

The first line creates a list of distinct student names.  The second line defines a function that will return a list of subjects for any given student.  The third line uses MAP to return the subject list for each student in turn.  This is where the Excel in-built functionality fails.  What I have done instead is use a function MAPλ that I wrote to overcome the problem.  The final line sorts the data alphabetically by student name, to give

It is complicated but the user need never look at it!