I found the solution for my problem.
It was a notation error to get the right data:
const { sanitizeEntity } = require('strapi-utils');
module.exports = {
async findOne (ctx) {
const { slug } = ctx.params;
const entity = await strapi.services['article-category'].findOne({ slug });
return sanitizeEntity(entity, { model: strapi.models['article-category'] });
},
};