Why can't I build anything on these tiles?

Seems like a bug, as usually the only parts of the map you cannot build on is the road, the very edge of the map and water.

Save your game and open the save file in a text editor. You can locate the map tile in the file using the X,Y coordinates and see what it says is there.

For example here are the first few tiles in one of my save games:

BEGIN Cells      
    BEGIN "0 0"        Mat PavingStone  Con 0.00000  END
    BEGIN "0 1"        Mat PavingStone  Con 0.00000  END
    BEGIN "0 2"        Mat PavingStone  Con 0.00000  END

Mat is a keyword for material, in this case they are paving stones. Con is the condition, or how much damage it has sustained; 0 means undamaged and 1 means it has been destroyed.

Find the cells you cannot build on and add the info to your question. Then you can edit the save file to change the material and reload the game.

These tiles are both water. As to why they have both been generated, I can only assume it's a bug. The lower tile looks different because it has a planning overlay on it.

They don't show up well in normal view because tiles on the border between two floor types never do show up clearly - but water is very distinctive in utility view:

Water tiles are not editable in the base game. Tony suggests a way to edit save files to correct this issue.

Some items can only be placed indoors - try placing a foundation, then place those items.