Rae from Shopify here. Thanks for posting about this error that your customers are seeing. I'm happy to help take a closer look into this and determine why this message is showing up on your site.
As a starting point, can you please confirm:
I'd also like to find out which theme you're using to check if this error is related to your theme. With this in mind, please visit the 'Online store > Themes > Current theme' section of your Shopify admin to find your theme name, and reply back to me here to let me know which theme you're using.
Once I hear back from you with the above details, I can help confirm the next best steps to get this resolved.
I wanted to check in with you and find out if this error is still appearing on your site? If so, please feel free to reply back to me here with the answers to the questions I included in my previous reply, and I'll be happy to continue lending a hand.