that could be because there is either no Pit AI line (like the AI fast line) or it has no markers for the pit section.. Both can be fairly easily fixed.
The other thing it could be is.. the pitlane has accidentally been given the Grass tag rather than the Road tag
To be honest i think it might be the third thing. if it was the pitline or pit markers that were missing, it would still treat it as just road, but if its saying you are offtrack and need to slow down, then that means i thinks it is either grass or sand.