I assume you are using the free app, please let us know if you're not using the free app.
The Mendix pricing page ( https://www.mendix.com/pricing/ ) shows:
"When we say free, we mean free. No hidden fees, no commitment, no credit card required. The free edition includes everything you need to design, build, and deploy demos, prototypes, or small applications. It includes a deployment environment for each application that provides access to up to 10 internal users. Just sign up, get started right away, and build as many applications as you want."
Information from sprintr: