We've seen an increase in this Instagram error due to changes in Meta (Facebook and Instagram's) API.
Instagram has updated their profile requirements so that certain fields are now deemed as required. When logging in to Instagram, you should see that a birthday is required through a pop up window. If not, please go to the profile settings and enter in a birthday, OR any other missing profile information such as a phone number, then retry the posts. This has resolved several issues.
If you are still experiencing issues, please log in to Instagram > Settings > then on the left hand side at the very bottom, you should see "Meta Accounts Center":
Then, click "add accounts". This is essentially linking Instagram to Facebook through Instagram's end:
Next, retry the post in Social Marketing, then refresh the page to see if it was successful.