Randbetween with the list of exception

Maybe a UDF like -

Function RandNumbers()

    Dim R As Long, MatchId As Integer
    
Here:
    MatchId = 0
    R = Application.WorksheetFunction.RandBetween(1, 99)
    On Error Resume Next
    MatchId = Application.WorksheetFunction.Match(R, Range("A1:A20"), 0)
    If MatchId > 0 Then GoTo Here:
    
    RandNumbers = R
End Function

A slightly more customizable version, where you can select the Min, Max Number & Range. Should be entered like this - =RandNumbers(1,99,A1:A20)

Function RandNumbers(Min As Integer, Max As Integer, Rng As Range) As Integer

    Dim R As Long, MatchId As Integer
    
Here:
    MatchId = 0
    R = Application.WorksheetFunction.RandBetween(Min, Max)
    On Error Resume Next
    MatchId = Application.WorksheetFunction.Match(R, Rng, 0)
    If MatchId > 0 Then GoTo Here:
    
    RandNumbers = R
End Function

Assuming all the A1:A20 values will also be integers in the range 1 to 99 you could use this "array formula" in C1

=SMALL(IF(COUNTIF(A1:A20,ROW(INDIRECT("1:99")))=0,ROW(INDIRECT("1:99"))),INT(1+RAND()*(99-SUM(IF(FREQUENCY(A1:A20,A1:A20),1)))))

Confirm with CTRL+SHIFT+ENTER