[Geschlossen] Category Post pages gives 404 error when translating into spanish

Background of the issue:
I am developing a site and trying to ensure all pages are translated into Spanish using WPML.

Symptoms:
Category Post pages give a 404 error when I switch to Spanish.

Questions:
Why do Category Post pages give a 404 error when switching to Spanish?
How can I ensure all pages are translated into Spanish without errors?