Can't select or edit any objects in a file

Hi, a client set me a simple .ai file (just some lines and text) but I can't select any objects in it. No matter what tool I choose, it just gives me a cursor that looks like a pencil with a little "no" sign:

I have tried unlocking all objects, creating outlines, saving a new version of the file, restarting, etc. but nothing seems to work. Please let me know if you have any insights.