That indicates that the Value of the cell in that column for at least one row was Nothing, which won’t work with ADO.NET. If you want to insert NULL then you have to use DBNull.Value. The way you’re doing things, that means explicitly checking for Nothing. You also should not be using AddWithValue, but rather calling Add and specifying the data type and, optionally, the size, e.g.

cmd.Parameters.Add("@KodeBuku", SqlDbType.VarChar, 50).Value = If(row.Cells("kode").Value, DBNull.Value)

I’m not 100% sure without testing but you may have to use CObj(DBNull.Value). Note that that If operator will return the first argument if it’s not Nothing and the second if it is.

A better option would be to not use a loop to call ExecuteNonQuery in the first place. You should create a DataTable with the appropriate schema and bind that to the grid. You can then use a data adapter with an appropriate InsertCommand to save all the data with a single call to Update. Every field in a DataTable contains DBNull.Value by default, so there’s no need to do any check for Nothing.