Without the exact dates we can only hazard a guess, but this looks like the message that comes up when there is no availability anywhere near the date in question. And by the looks of it the last First on sale from LGW to UVF is on 31 December. You could try that date (for 1 passenger) to see if it works. CW has a bit of availability in January 2019 but not much and not viable for a return starting from the UK. The usual advice is to book in the cabin you are actually prepared to travel, then keep an eye on how things turn out. For CW it's one of those routes where the 355-day mob will turn up, so it's not easy to get seats on that route.