Are you on Windows? At first launch after Windows install, Cura is sometimes unable to connect to the engine. This can cause the slicing to remain at "preparing to slice". Restarting Cura should correct the connection problem.
This is an example on how to upload your entry.
This is not an entry to the contest.
Print the smallest molecule you can!
Do you want to participate in the contest and win a free special edition Olsson Block?
You can participate here!