The sentence can be written with or without 'the' 'All the preparations are complete for the royal wedding.' 'All preparations are complete for the royal wedding.' (both are correct) 'The' can be used when referring to the preparations for a specific occasion. The noun 'preparations' is always plural when describing things that are done to make something ready. When we talk about preparation as an activity or process, it is uncountable. 'The festival involves a lot of preparation.'

You can also say, all preparations have been completed, or everything is ready, or preparations are done..

You better say: "All preparations are complete" ( made or done).