If you're sure that the DNS records are set correctly, then you should reach out to Shopify's support team directly for help with this. Sometimes (very very rare that it happens), the SSL certificate can get 'stuck' as pending, and a Shopify employee just needs to reset/reissue it. If that's the case for your domain (assuming your DNS records are correct), then it should only take them a few minutes to fix it for you (and then an hour or so for the changes to take affect).
You can lookup your DNS records using whois search: https://who.is/
Or else via your domain provider, where you can edit the DNS records directly.
This doc outlines what the records should be: https://help.shopify.com/en/manual/domains/add-a-domain/connecting-domains/connect-domain-manual
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