You can’t attach a screenshot, if the file is created or saved locally you can use windows explorer or another file management program to drag and drop your image into your post.
That is what I did for my screenshot.
I didn’t get the error message you described only the text in my image error code 521.
This is what error code 521 represents:
Error code 521, “Web server is down,” signifies that a website’s server is unable to connect with Cloudflare, the CDN (Content Delivery Network). Essentially, Cloudflare is trying to reach the website’s server, but the server is either offline or refusing the connection. This often happens when the server is experiencing downtime or when its firewall is blocking Cloudflare’s requests