It looks like you're not querying against the database but against an existing dataset. For this reason you can't use the
LIKE
as if you had run it directly on SQL, but you can use an appropriate C# version instead. For example:
Rows
.Cast<DataGridViewRow>()
.Where(row => row.Cells["R.b. RN"].Value.ToString().ToUpper().Contains("KO"));
This essentially replicates the behaviour you're expecting from the
LIKE
keyword. If you want to do this from the SQL then you'll need to amend your SQL query to leverage it.