What data type is used to store Man_ID value?
If it's string, then try this:
Dim comm As New SqlCommand("Select [Man_ID] from Manager where[Man_Id] = '" & manid.Text & "'", conn)
If it's integer, try this:
Dim comm As New SqlCommand("Select [Man_ID] from Manager where[Man_Id] = " & manid.Text, conn)