I tried to send invoices using sendmail and smtp. but any mail is not sent
Please try using the test email button in the email settings. If there's an error, it'll display it there.
The next places to check are the email delivery queue and to validate cron is setup and running correctly.
cron is not running correctly. how can i fix it
I can't really help you without having any more info other than "not running correctly".
Check the basics: