SHA256 in .Net -- Incorrect hased value

I have to hash the value using SHA256 algorithm.
Value to be hashed: NIKKI556538684c657633314734696a4d64754736496e62772a2atest
Hashed value should be: 88679eb665c100b66c04770e87f83b1692df2f027c16adb198addfe3bdbe6c1d
The code that I am using to hash is as below:
Dim hashVal As String = ByteArrayToStr(HashValue(StrToByteArray(concatValue)))
Private Function HashValue(ByVal data() As Byte) As Byte()
        Dim hash As New SHA256Managed()
        Dim result() As Byte
        result = hash.ComputeHash(data)
        Return result
End Function
Public Function StrToByteArray(ByVal str As String) As Byte()
        'Dim encoding As New System.Text.ASCIIEncoding()
        'Return encoding.GetBytes(str)
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