If you receive the error from the title, when trying to upload images to Instagram, please follow the following solution:

Instagram used to be all about the square image. However, you can now upload landscape (horizontal) or portrait (vertical) photos as well. Here are the best sizes for Instagram’s three image types:

• Square Image: 1080px in width by 1080px in height (Aspect ratio 1:1)
• Vertical Image: 1080px in width by 1350px in height (Aspect ratio 4:5)
• Horizontal Image: 1080px in width by 566px in height (aspect ratio 1.91:1)

Square

Vertical

Horizontal

 

You should automatically rescale your images to one of the above sizes, to be sure that all published images are automatically posted to Instagram. To do so, you can use a free plugin, example: https://wordpress.org/plugins/imsanity/