It’s absolutely possible to start with Clozemaster as an absolute beginner, but it’s not the easiest way to learn a language from scratch. Nor the most efficient way (probably).
I can say that because I started learning Spanish with Clozemaster having had no prior knowledge of Spanish at all.
Some guy who already knew multiple other languages studied Japanese from scratch with Clozemaster, and look at his Clozemaster profile!
If you need a lot of hand-holding or a structured approach, starting from zero with Clozemaster is not for you. If you’re the “building the airplane as you fly it” type of person, then try it. What’s the worst that could happen?
That you realize that you need to buy a grammar book to study in parallel to Clozemaster?
Then nothing was lost.
Learning Spanish for a while now, I still struggle with the grammar. The Spanish words look familiar by now and I can select the correct solution from the “multiple choice” mode, but I cannot yet do “text input” as I can with my other language which I started using Duolingo before I started practicing it with Clozemaster. (Although I think that multiple choice is way too easy.) Had I solely relied on Clozemaster, I would still not know when to use the Spanish conjugations “hago”, “hace”, “hice”, or “hizo”, as an example. What I would know by now is that they all stem from “hacer”, but you definitely need to create your own flashcards (for example, in Anki) to practice verb conjugations on your own. And study the grammar using resources other than Clozemaster. For vocabulary, Clozemaster remains king though. 
If you find a good Spanish grammar resource, post it in this topic.
The previously mentioned guy who learned Japanese had let Reddit decide which language he should learn next (they chose Japanese). Before he started, he analyzed all the Kanji in the Clozemaster course. At the 100-days-mark, he gave a status update. He was very happy with his progress, having started Japanese without prior knowledge. In his opinion, Clozemaster is an awesome tool to quickly bring you (from zero) to the level at which you can watch japanese shows with subtitles. Clozemaster brought him to the point of having a reading vocabulary much faster than any course or book could have done, he thinks. And once you can understand/read Japanese subtitles, the real fun begins in case you enjoy anime. And, regarding Japanese, you definitely need to complement Clozemaster with watching media and/or reading books to develop a natural way of speaking Japanese because the Japanese sentences on Tatoeba (Clozemaster’s source) are often very weird, according to him.
All in all, what you’re considering to do is absolutely doable.
If you’ll enjoy the approach of stumbling your way through sentences without understanding the grammar, you have to answer for yourself.
Whether this approach is the most efficient way to wholly learn a language is debatable.
To repeat my point: Starting with Clozemaster and no prior knowledge, you won’t be able to speak Spanish with someone because you won’t know if you should say “hago”, “hice”, or something else. But when you’ll read “hice”, you’ll recognize the infinitive “hacer” and know from context that somebody is or was doing something.
So if you should first go through a grammar book before starting with Clozemaster probably depends on what your goal is:
- to speak Spanish in a dialog
- to read a Spanish book or subtitles