Proporties of mixtures has no set composition
must have more than one state of matter
must have more than one substance

What mistake did Dominic make?

The title should read “Properties of Solutions” because some mixtures do not have all of the properties listed.
There is a definite recipe to make each mixture, so the composition of a mixture is set.
Although it is possible to have more than one state, it is also possible to have only one state.
A single substance can be used to make a mixture if the substance is composed of more than one element.