A couple of issues

[rnkn]

You need to autoload the mode definition so package installations will see todotxt-mode. See: https://www.gnu.org/software/emacs/manual/html_node/elisp/Autoload.html

Also, you need to (require 'org) in order to use it in the function todotxt-add-due-date.

You may also want to add a hook when changing major mode to delete the overlays.