Could you quote your code in the future? It makes it easier for people to run your code, and increases your chances of getting a quick response.
You seem to be using a very old version of JuMP (@defVar hasn’t been a thing for a long time). Which version of JuMP are you on? Which version of Julia are you on? FYI it’s not recommended to install Julia via apt-get as it’s changing so quickly (just a guess); just download 0.6.2 from Download Julia.
JuMP doesn’t define ^(::Float64, ::JuMP.Variable). This is because the ‘default’ interface (using @expression, @objective, and @constraint in the latest version of JuMP) only supports affine and quadratic expressions. Using a JuMP.Variable as the exponent cannot result in such an expression. you may want to use @NLexpression /@NLobjective/@NLconstraint instead