Relational Model Design - Entity with one attribute
Yes, but.

If an entity only has one attribute, then that attribute must serve as an identifier for the entity.  Typically, if only one attribute is found for an entity in the given data stream, then another attribute is "invented" for purposes of identifying.  It's often called "id", it's often an integer, and it's often populated using the autonumber feature of the database system.

When you are doing analysis, it's important to distinguish between the features that were given to you in the problem statement, and the features you added in order to make sense out of the problem.