You are printing the column's names of the datatable returned by GetSchemaTable, not its values, also I suggest to use a StringBuilder and write everything when you exit from the loop
StringBuilder sb = new StringBuilder();
foreach (DataRow row in schemaTable.Rows)
{
foreach (DataColumn column in schemaTable.Columns)
{
sb.AppendLine(column.ColumnName + ":" +row[column.ColumnName].ToString());
}
sb.AppendLine();
}
File.WriteAllText(@"C:\Users\Manuela\Documents\GL4\WriteLines.txt", sb.ToString());