How to re-open an issue in github?

Github has very simple rights/privileges (and even simpler management for them).

If you are not a collaborator of a repo, then (in regards to issues)

• you can open issues
• you can comment on all existing issues (open or closed)
• you can close your own issues
• you can re-open your own issues if you closed them yourself
• you cannot close or re-open issues opened by someone else
• you cannot re-open your own issues if a repo collaborator closed them

The situation you report falls in the last case, so you cannot reopen it. You can ask in the issue thread if a repo collaborator can reopen it, or you can open a new issue and reference the old one (by mentioning its number preceded by a hash sign, e.g. #123).

It appears that they prefer that users NOT create duplicate issues. From the GitHub issue template on 9/1/2020 (from the existing bug's page click 'New Issue', 'Get Started'):

Before filing, PLEASE check if the issue already exists (either open or closed) by using the search bar on the issues page. If it does, comment there. Even if it's closed, we can reopen it based on your comment.