Heya! Be sure you are setting your position with a //wand correctly. If the white_stained_clay is not working, try using its number code (which is 159) or its "Google" name ID: stained_hardened_clay
Also, if you want a specific colour of something like stained clay, you separate the ID of the block and the colour value with a colon, so for example, yellow stained clay would be //set 159:4
Since it’s not mentioned earlier I will let you know that if it’s not enabled you can press f3+h to toggle the advanced tool tips which shows the id for all items when you hover them in creative mode! :)