You need to have a custom domain in order to apply to WordAds. From there, if you reach the minimum amount of traffic, you might be accepted into the programme, but please bare in mind it requires thousands of views to make meaningful revenue each month.
Sites on the Premium or Business Plan are automatically accepted into WordAds. Learn more: https://wordads.co/faq
Sadly though, without a paid custom domain, it isn’t possible to become part of the WordAds programme.
Let us know if you have any further questions or issues!