Thank you for bringing this to our attention! I have taken a look at the most recent social post campaign in your account and was able to see this same error message. In fact I can also see there is the option to retry sharing your post. Does clicking this retry button allow your image to post on Instagram?
Because Instagram is primarily about sharing images, I would even suggest trying with an image that does not have a lot of text on it. We also have our upper level of support available by phone who can troubleshoot further if you're unable to post after trying either of these workarounds. These social post campaigns are a new type of campaign that we offer our users, so we want to make sure this is looked into for you!