What is the meaning of 'Unknown Image Error Provided og:image URL, [your-image-link] encountered an unknown error' on Facebook debugger?

It doesn't mean that the image is having a problem. 

It's because Facebook debugger is not stable. When you scrape for the 1st time, it may not be able to read the image from Shopify. However, when you try to press the "Scrape Again" button for the 2nd time, the message will be gone. The correct social link preview will be shown.