By default, students cannot reply to Announcements. This is a setting that we enabled at Dominican to more clearly differentiate Announcements from Discussions and which we felt best represented the majority of how faculty viewed Announcements as something that should not be replied to.
However, we know that some faculty may prefer students to be allowed to reply to Announcements. Somewhat confusingly, your view of a new Announcement seemingly will allow a Reply. However, this is not the case, as you would see by heading into the Student View. You must instead enable the ability for students to reply to Announcements within your course. This can be enabled for your course by following a few extra steps, outlined below.
First, head to the Settings menu area of your course site, and be sure that the Course Details tab is enabled. Next, scroll all the way down to the bottom of the screen, and click on the more options link. That will expand the options, and if you scroll down some more, you will see an option that is enabled called Disable comments on announcements, which is our University default setting:
Be sure that you uncheck that checkbox, and then click the Update Course Details button at the bottom.
This will NOT, however, disable comments from all Announcements in your course automatically. Instead, this allows you to choose which Announcement(s) for which you want students to reply.
To enable the setting to allow replies, create your new Announcement first, as shown on this Canvas documentation page: How do I make an announcement in a course? From the Announcements landing page, click on the Announcement that you wish to modify the settings for. You should see a settings icon on the right of the screen, next to the Edit button:
Click the option for Open for Comments, as shown above, and your Announcement can now be replied to by students.