In order for a project to be an assignment, it must be in a space. Are you using a space for your projects? If you are, you can find the option in the project's Access tab:
Open the project.
Click the Project Settings button (the gear icon in the upper right)
Click the Access panel
Under "Who can view this project", make sure the "Everyone in ..." option is selected
Check the "Make this project an assignment" checkbox
Hi, I understand now. The problem was that I have the project in my workspace not in a new space. Then, the problem is the number of students I can have with an assignment. Well, I understand that it is the free version.
Sorry for my doubts, and I'd like to make it clear that I think Rstudio Cloud is great.