You can find Dojo either within the Home tab (Review Tools) or within Explore (Review & Progress) and you can start using it after learning 50 words.
Dojo is based on a Spaced Repetition System (SRS), which is an optimal learning algorithm that predicts when you need to practice a specific word to maximize the probability of you memorizing it.
The message "no words available for optimal learning" means that the system estimates there are no words you need to practice for memorizing at that time. Based on your personal SRS score, you know the words you have learned within Drops well enough and there's no need for you to review them at that time.
Please note that Characters do not count as words for Dojo.