Good morning @CeciliaWhite and @bontomasin ...
In doing a quick search here in the Community, I found the following discussion topic that has a handful of suggestions: How to give all students the same comment... - Instructure Community (canvaslms.com).
From @KristinL:
I know it's not the same as an assignment comment, but using the Message Students Who... feature from the Grade Book or New Analytics would get an individual message to everyone who submitted. While not attached to the assignment in the same way as a comment, it would save you a lot of time.
Here's the lesson that illustrates the steps:
How do I send a message to my students from the Gradebook?
From @Stef_retired:
when I was teaching, I used [edited to add: the free version of] Phrase Express—Autotext - Boilerplate Template Manager - Text Expander—for this purpose.
From @chadscott:
Another option to look into would be Annotate Pro - It works across Canvas, Google Docs, etc. and is completely free for teachers.
Annotate PRO | Overview - 11trees
Hopefully some of these suggestions might be of help to you. Let Community members know if you have any other questions about this...thanks!