Isn't there a Ringworld in the OTU canon?
Yes, but it is a dead ringworld, it is a curious artifact and not much else, a lot like the Dyson Sphere that made its appearance in Star Trek: The Next Generation, which served as a giant mouse trap for starships. I wanted a fully functioning ringworld in a universe where it makes sense to have one, enter Alternate Earths, in a normal Alternate Earths campaign you are limited to alternate earths that are actual planets, something like a ringworld would be impossible to travel to as parachronic conveyors aren't spaceships. Traveller has spaceships so its parachronic technology would be space based and this opens up whole new possibilities. The Imperium's technology is based on found wormholes from the Ancients, so if there is not a wormhole leading there, they can't go there. Each wormhole is unstable and only allows passage when incorporated as part of a modified jump drive, for an instant the jump bubble inflates the wormhole so a starship can enter, this is the wormhole drive.
The cost of a wormhole drive is similar to a jump drive, it requires jump furl and passage through it takes a week, that week inside the wormhole is time that is only experienced by the crew and passengers of the starship, as their local arrow of time has been turned sideways.
At the present time, the imperium only has one wormhole for each universe it can explore, so for the time being only one starship can visit a parallel universe at a time. It could be a large starship, it would require a more powerful wormhole drive, but only one starship at a time can use a given wormhole, so the Imperium can only send one starship at a time to investigate the ringworld, if it wants to send another starship then the first one it sent has to return and its wormhole transferred to a new starship. Fortunately wormholes are very elastic, it simply requires more power to stretch one around a larger starship. As the research base on Luna has limited funding, it can only at this time afford a fleet of starships of 800 displacement tons or less, most of this research is unknown to the public, the Imperium is afraid that this technology could be weaponized, so they keep it secret and low profile so the enemies of the Imperium don't get wind of it, one of their starships has already been stolen by a Solomani Agent, this was 25 years ago, and he has never been heard from again. Some starships just never return from their missions and no one ever learns what happened to them, that is a risk every crewman takes when exploring a parallel universe, the only way to and from that universe is by that one starship, if it gets destroyed, the wormhole and that universe is lost. The Imperium is aware of two other societies that can travel to parallel universes, Homeline and Centrum, their technology doesn't suffer from those limitation. The relations between homeline and the imperium are cordial, but Homeline doesn't trust the Third Imperium because its an empire, and thus doesn't want to give it its parachronic technology because the Imperium might use it to conquer Homeline's Earth.