If you are talking about bridge: your problem here is obviously the length. Pak128 tries to keep length limits realistic.
If you are talking about elevated ways: the problem here is that depth of lakes is bugged since they have been added to the game, so it's impossible to build elevated ways on lakes, although it is still possible on sea (given that its depth is low).
If you want to use bridges, you have to terraform small patches of land on the lake to build several bridges between each.
If you want to use elevated ways, you can still try to terraform the lake so as to reduce its depth (possible but difficult and very expensive ! Better save your game before trying).