I am experimenting with sending emails with attachments and get the this error message when I run the following code:
System.EmailException: SendEmail failed. First exception on row 0; first error: INVALID_ID_FIELD, Invalid template id.: []

Suggestions?

public class EmailManager {

    // Public method
    public void sendMail(String address, String subject, String body) {
        // Create an email message object
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        String[] toAddresses = new String[] {address};
        //List<String> docid=new List<String>{'015o00000017GD1'}; // Give the document ID here
        mail.setToAddresses(toAddresses);
        mail.setSubject(subject);
        mail.setPlainTextBody(body);
        //mail.setDocumentAttachments(docid);
        mail.setTargetObjectId('003o0000005g1v8');
        mail.setTemplateID('01Ho0000000TQmj');
        mail.setSaveAsActivity(false);
        
        // Pass this email message to the built-in sendEmail method 
        // of the Messaging class
        Messaging.SendEmailResult[] results = Messaging.sendEmail(
                                 new Messaging.SingleEmailMessage[] { mail });
        
        // Call a helper method to inspect the returned results
        inspectResults(results);
    }
    
    // Helper method
    private static Boolean inspectResults(Messaging.SendEmailResult[] results) {
        Boolean sendResult = true;
        
        // sendEmail returns an array of result objects.
        // Iterate through the list to inspect results. 
        // In this class, the methods send only one email, 
        // so we should have only one result.
        for (Messaging.SendEmailResult res : results) {
            if (res.isSuccess()) {
                System.debug('Email sent successfully');
            }
            else {
                sendResult = false;
                System.debug('The following errors occurred: ' + res.getErrors());                 
            }
        }
        
        return sendResult;
    }

}