You have not posted enough information for us to answer your question. For example, the way to write f(t+1) in Julia is usually as a function call, i.e. f(t+1). However, in a JuMP model with discrete variables you may want an array reference like variablename[t+1]. Or it could be other things depending on your code.
So if you want help, please include a small but relevant section of your code. And please format your code using backticks, like this.