A launch rarely fails because of one big problem. Usually it is a dozen small things, each easy to forget on its own.
Technical
HTTPS is configured with a redirect from http
One canonical domain is chosen: with or without www, never both
robots.txt and a sitemap exist and do not block what matters
Custom 404 and 500 pages instead of a blank screen
Content and SEO
Every page has a unique title and description
Exactly one H1 per page
All images are compressed and have alt text
Links are checked — none point at localhost
Analytics and forms
Trackers are installed and actually see traffic
Forms really send mail instead of failing silently
Database and file backups are configured
We run this list on every project before handover — and still find an item or two every single time.