Among the main reasons why a site could be omitted from a search are:
Launching it on a trial account. Make sure you’ve chosen a Shopify tier to actually make your store indexable.
You left the site password-protected while it was undergoing maintenance.
Some redirects have not yet been put in place.
Most of these should be eliminated if you follow our Shopify SEO best practices, so we’ll add that it’s possible for a website to have too few keywords. It’s not a very likely case for a store, but do check to confirm you’ve added enough content for the search engine to latch on to.