You are missing some PHP extensions:
IMAP, intl, also may want set timezone
"PHP Mail Function:@ mail() returned failure"
it's under Emails>Settings
Outgoing Email: Default email only applies to outgoing emails without SMTP setting.
Default MTA: from PHP to Your support email
Also, osTicket v1.14.2 does not support PHP version 7.4, so you should downgrade it to 7.3