Sangapi Airport does not have a lounge, and for a small highlands airstrip in Papua New Guinea, that is simply the nature of the place rather than an oversight. This is a basic strip serving a remote part of the PNG highlands, not a terminal built for connecting passengers or premium amenities, and nothing about its size or setting points toward one being added.
The bigger factor shaping travel through Sangapi is weather, not terminal design. The Papua New Guinea highlands are known for cloud cover and fog that build through the morning, and flights into small strips like this one are frequently timed for early morning windows before conditions close in, sometimes shifting or cancelling later in the day. That reality matters more to your trip than any lounge question, since delays tied to highlands weather are the more common disruption travelers actually face here, far more than the absence of a waiting room amenity.
For anything resembling a lounge, look toward Mount Hagen instead. Kagamuga Airport, the main highlands hub, handles the bulk of connecting traffic in this part of PNG and is a far more realistic place to expect any airline or premium passenger amenities than a small strip like Sangapi. Even there, do not assume an elaborate facility; highlands aviation in PNG is built around reliable connections more than terminal comfort, and that priority shapes the whole regional network, Sangapi included.
At Sangapi itself, plan for a basic strip with minimal shelter and no food or lounge amenities to speak of. Build real flexibility into your schedule around highlands weather patterns rather than around lounge access, since a delayed or rescheduled flight is the more likely event to plan for. If your itinerary routes through Mount Hagen, treat that leg as the point to reassess any lounge or waiting-area expectations rather than Sangapi, and keep a buffer for weather on both ends of the journey.