419 Page Expired" when I submit a form

A 419 means the security token that comes with every form did not match the one the server expected.

Try this, in order:

1. Reload the page and submit again. A page left open for hours expires.
2. Make sure cookies are allowed for the site.
3. Check APP_URL in .env. It must be the real address you use, including https:// and www or not. Then run php artisan optimize:clear.
4. Make sure storage/framework/sessions is writable.
5. Behind Cloudflare or a proxy, set SSL mode to Full and make sure the proxy sends X-Forwarded-Proto.

See also Common server problems.