To decorate choco, go to restaurant on 13th February and talk to the restaurant owner! You'll then be able to make homemade choco and decorate it :D
And yeah, it is fine to pre-order in another computer. As long as you login with the itch.io account you purchased the game with, you should be able to download the full game in any computer you have :)
Can you show screenshot of the exception traceback?