I understand that this card in the whole Dreamscape map... but what about its effects? It says that 3 symbols have no effect. Does that mean, that the spaces with stairs (one of the symbols) are not valid? Or is it just a reference for some effect, that will take place later?
It's a reference to effects that will take place later (and you can freely travel to those spaces
Those symbols don't mean anything until the game gives them rules. Once the game gives you special rules for stairs/manhole covers, Caution Repairs tells you to ignore those special rules.
For now, you can just treat them like normal spaces.