Instagram will display this error if your content is not in the
correct format for Instagram. Many times this means the photo or video
you are trying to post is not in the correct aspect ratio (dimensions).
Instagram the best formats for Instagram's Timeline are 1:1 (square), 16:9 (landscape), 4:5 (portrait). You can use Greatly Social's photo editor with preset crops for Instagram. Follow this guide on How to Crop Photos on Greatly Social.