why do sims decide where i place a mural

@ca3a3861 Where are you trying to make a mural? Do you have the City Living pack? If you look in build mode in Activities and Skills you will see that there is a floor and a wall 'DIY' mural, plus two garage door versions that you can use/add yourself to editable lots/areas/builds. These also seem to be what is used in the public areas where murals can be done in that city (San Myshuno - the one that comes with the City Living expansion pack). They do have specified parameters/size, though, and the sim will not make a mural somewhere that does not have one of these added to the area as far as I know. Think of it a bit like them painting on a canvas, but larger and with a few more possibilities.