try like below if you want to avoid null values
DataTable dt = new DataTable();
dt = FnTableIDInsert();
foreach (DataRow dr in dt.Rows)
{
string value = dr["tableid"].ToString();
if(value!=null && value!=string.Empty )
{
SqlConnection connn = new SqlConnection(ConnectionString);
connn.Open();
string sqlselect = "INSERT INTO tbl_clientcolumnnames(tableid) values("+value+")";
SqlCommand cmdselect = new SqlCommand(sqlselect, connn);
cmdselect.ExecuteNonQuery();
string sqlselect2 = "INSERT INTO tbl_Intellectcolumnnames(tableid) values(" + value + ")";
SqlCommand cmdselect2 = new SqlCommand(sqlselect2, connn);
cmdselect2.ExecuteNonQuery();
}
}