It looks like you might be on an (old) beta-release of JuMP. Run
] rm JuMP
] add JuMP
Once you’ve done that, the following should work.
using JuMP
model = Model()
@variable(model, x)
all_variables(model)
# output
1-element Array{VariableRef,1}:
x
In future, please read Please read: make it easier to help you.