For one, you are having issues with the Dashboard > Information page. It's not loading all the way so it's not showing all the information on that page. I'd suggest reviewing your logs (general server logs, webserver error logs, PHP error logs, MySQL/MariaDB error logs, osTicket System Logs, Browser Console logs, etc.) for any related errors.
As for the "valid email" error, it seems you are having DNS issues and it can't verify the MX records on the gmail.com domain. You can turn off email verification by going to Admin Panel > Emails > Settings, uncheck the box next to Verify Email Addresses, and Save Changes. However, this should only be a temporary solution until you get the DNS issues resolved.