It requires infinite time in addition to infinite energy. It's also not a "just", since it needs an engine which can always overpower the breaking effect of the occasional particles of intergalactic space. Even in pure vacuum, there's still an interaction with the 3K background background radiation. (For cosmic particles this is the Greisen–Zatsepin–Kuzmin limit.) Beyond that, there may be vacuum Cherenkov radiation to overcome.
Infinite energy doesn't help if your engine produces waste heat, unless you can also cool infinitely fast.
Infinite energy doesn't help if your engine produces waste heat, unless you can also cool infinitely fast.