Hello there, @115694394375500 ...
Without being able to see your school's Canvas environment or your course(s), it's hard to diagnose exactly what is happening with that particular discussion topic. However, I assume that you are using the instructions outlined in this Guide to try and reply to your discussion topic: How do I reply to a discussion as a student?. It could be quite possible that your instructor is not allowing comments past a certain date. In your course, do you have a "Discussions" course navigation button on the left-hand side of your screen? If so, click on that button and see if the topic is listed under a heading called "Closed for Comments". If that topic is listed under that heading, then you and your classmates can no longer post comments for the topic.
There is a second possibility, too. If your instructor has not put that topic in the "Close for Comments" heading, your instructor may have set "Available From" and "Until" dates on that topic. This Guide is meant for instructors, and in the section "Add Discussion Options", it describes more about these two date fields: How do I create a discussion as an instructor?. The part I want to point out here is (highlighting is my own), "Before the Available From date, students will only be able to view the discussion title. After the Until date, students can view the discussion topic and all responses but cannot add or edit any responses."
With all this being said, I would recommend that you reach out to your instructor to have a conversation about this. Your instructor may have purposely closed off the discussion topic and made replying unavailable to you and your classmates. You can contact your instructor in Canvas by using this Guide: How do I get help with Canvas as a student?.
I hope this will help a bit. Sing out if you have any questions...thanks!