Hi @JOSEZAGAL,
I checked some of our prior-term courses here while acting as a teacher, and I can confirm that the +Announcement button is not there. I'm fairly sure that your local Canvas team likely changed the way they set up dates for terms in Canvas. We only allow our teachers full-access to a course for one week after the term ends. In that time, the +Announcement button does appear to show. After hat period, the course automatically goes into read-only/soft-conclude mode based on the term settings, and the +Announcement button is gone.
We intentionally have things set up this way to help contain thing within the term and make sure students aren't getting messages from courses they took years ago (this was happening in our past LMSes). We do allow that one-week period as a compromise for things like you're trying to do though.
Your local Canvas team should be able to control this, so perhaps having a chat with them may get them to alter the settings if this is a new policy they are trying out for the first time.
Good luck, and I do hope this info was helpful!
-Chris