Lisa here from Shopify Support.
Thanks for sharing. It could possibly be due to the text overlay, the updated Facebook text overlay tool approves it here, although, on the more traditional checker here, it says there's 28% text overlay, exceeding the 20% limit, as shown in this screenshot.
Were you trying to create an offer? Here are more details about creating offers on Facebook, and the design guidelines:
Recommended image size: 1,200 x 628 pixels
Image ratio: 1.9:1
Offer title: 25 characters
Text: 90 characters
Your image should include a minimal amount of text.
Along with a more general guide for all types of Facebook Ads.
Hope this gives more insight as to why Facebook refused your ad & fingers crossed after some adjustments they will approve it for you soon!
- Lisa ?