Check from "Issues" tab whether an issue for the bug has been already opened or closed.
You will find a button called "New issue" from "Issues" tab which will redirect you to a new window.
Clicking "Get Started" will redirect you to a window where you can issue a bug. You will have formatting tools and file upload available as well as ability to preview your text.
After that, you can click "Submit new issue" from the bottom.
You can also report a bug in #bug-reporting in Etterna Dev Group Discord.