Low graphic settings will block several more things to be honest. Just changing to medium or better should solve that, but these are done for a reason, if your computer struggles to run it on low, you shouldn't try to change it to medium.
Does it mean I have lower level computer, i.e graphic card that cannot generate foliage or I have to change something in settings?
If I'm not mistaken, it means that while all PC's can technically generate foliage, if the graphic settings of the game are set to their lower options, the game will not generate foliage as it is a big performance drain. I'm pretty sure 'Terrain Quality' is the setting that has to be turned up for foliage to be generated.