The attachments look like this and a typical screenshot is larger then the allowed file size for attachments. So your post didn't contain the screenshot you attempted to upload.
It is better to use a website you feel comfortable using.
Imgur is one example but it's not the only website. I sometimes put a screenshot on Discord and use a link to the image for the forums. You can do it several ways.
You can try to resize the image but don't go below 50% smaller. Try for 75% the original size first and see if you can attach the image. Just try your best