The parameters need to be an array. If you want to use an array that has a bit more structure, I’d suggest using ComponentArrays.jl, which has an example on this:
https://jonniedie.github.io/ComponentArrays.jl/dev/examples/DiffEqFlux/
Or if you want to have multiple different arrays for the parameters, you can use the Flux style for the optimization demonstrated at:
https://diffeqflux.sciml.ai/dev/examples/neural_ode_flux/
(However, note that the Flux style does limit the available optimizers)