While running test class.. I am continuously getting this error

Invalid type: Schema.SingleEmailMessage

#Code:-

@isTest

public class TestContactEmailTrigger {

    @isTest

    public static void testTrigger() {

        // Create test data

        List<Contact> contacts = new List<Contact>();

        for (Integer i = 0; i < 5; i++) {

            Contact contact = new Contact(

                LastName = 'TestContact' + i,

                Email = 'test' + i + '@example.com',

                Message_sent__c = true

            );

            contacts.add(contact);

        }

        insert contacts;

        // Perform DML operation to trigger the email sending

        Test.startTest();

        for (Contact contact : contacts) {

            contact.Message_sent__c = true;

        }

        update contacts;

        Test.stopTest();

        // Verify the emails are sent correctly

        List<Messaging.SingleEmailMessage> sentEmails = [SELECT Id, ToAddress, Subject, PlainTextBody FROM Messaging.SingleEmailMessage];

        System.assertEquals(5, sentEmails.size());

        // Check the content of the email

        for (Integer i = 0; i < 5; i++) {

            Messaging.SingleEmailMessage email = sentEmails[i];

            Contact contact = contacts[i];

            System.assertEquals(contact.Email, email.getToAddresses()[0]);

            System.assertEquals('Important Update', email.getSubject());

            System.assertEquals('Hello, \n\nThis is an important update regarding your contact information.', email.getPlainTextBody());

        }

    }

} #Salesforce Developer