Issues are disabled by default in forked repositories. However, you can enable them by following these steps:
Navigate to the Settings page of your forked repository.
Click on the General tab on the left-hand side.
Scroll down to the Features subsection.
Check the box for Issues to enable them.
Additionally, if you want to add an issue to someone else's fork, you would need to ask the owner of that fork to enable issues in their repository settings. As a workaround, you could fork their repository yourself and submit a pull request, which allows you to leave comments.