Task to delete users without mount points in /home (using register and when)
Several problems in your playbook.


loop is basically expecting a list, not a string containing a variable name. You need to expand your variable with a jinja2 expression => loop: "{{ some_list_variable }}"
It will be hard to give you the correct syntax without seeing the exact data structure of your result var but... Typically, the register from a loop contains a results list of hashes, each containing info about the ran command for each loop iteration. This is your base loop for the second task => loop: "{{ del_users_reg.results }}"


If you still have trouble fixing your playbook, update your question with a debug of the registered var and I will update my answer accordingly.