The certificate for this site is not valid

[Rick1969]

I have had a new domain since Nov 2023 ( 3 months) and all has been great. Suddenly I have had customers and family letting me know they get a security warning when trying to access my site (  Home - VIP Essentials Soaps & Candles (vipsoapcandle.com.au) ) saying "Your connection is not private"  and "The certificate for this site is not valid".  Tried Shopify support with no luck, their solution was to tell my customers to ensure their PC has the latest updates and their have the latest browser version. Like that is ever going to be a realistic solution. I was able to use TeamViewer to take over a family members PC who was also having the problem. I checked and they have the latest Windows version and Edge version but still same errors. I downloaded Chrome and same issue.

 Any help would be appreciated, as a customer's getting this error will NEVER trust my site, but it is only happening to 4 out of 10 people I asked to test. But 40% failure rate is significantly bad for customer trust.