My 80% certain answer: the Collabora repos are not added to your APT for some reason. There should be a /etc/apt/sources.list.d/collabora.list with the content
deb https://www.collaboraoffice.com/repos/CollaboraOnline/CODE-debian10 ./
(or smth to that effect)

If this is there, maybe you forgot to run “apt update” after you added the repo.

Good luck, hope this helps!