[code=vb.net]

Public Class Form1
Dim m, n, i, j As Integer
Dim arr(20) As array1

Structure array1
Public a()() As Array
End Structure

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Button1.Enabled = True
Button2.Enabled = True
Button3.Enabled = True
Button4.Enabled = True
End Sub

Private Sub Button5_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button5.Click
m = InputBox("Number of Employee")
n = InputBox("Number of Product")
j = 0
For i = 1 To m - 1 Step 1
// error in this line as value of string type cannot be converted to system.array
arr(j).a(i)(0) = Val(TextBox1.Text)
'MessageBox.Show(arr(j).a(i)(0))
Next i
End Sub
End Class

[code=vb.net]

Public Class Form1
Dim m, n, i, j As Integer
Dim a(20)() As Integer

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Button1.Enabled = True
Button2.Enabled = True
Button3.Enabled = True
Button4.Enabled = True
End Sub

Private Sub Button5_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button5.Click
m = InputBox("Number of Employee")
n = InputBox("Number of Product")

For i = 1 To m - 1 Step 1
'' error in this line as Object reference not set to an instance of an object.
a(i)(0) = InputBox("ENTER")
MessageBox.Show(a(i)(0))


Next i
End Sub
End Class
[code]

You haven't assigned the number of elements in the second dimension of the array:
Dim a(20)() As Integer
either
Dim a(20)(10) As Integer ' For example 10
or later in the code
ReDim a(20)(10)
or from a variable
ReDim a(20)(SomeVariable)