Hi,
Velvet is not available at Galaxy Main https://usegalaxy.org. Instead, we recommend Unicycler for small genome assembly.
GTN Tutorial: http://galaxyproject.github.io/training-material/topics/assembly/tutorials/unicycler-assembly/tutorial.html
Maybe you are working at a different public Galaxy server? Check/compare the URL. Not all public Galaxy server administrators check posts here, instead contact them directly. Contact information is usually at the home page of the server and sometimes also here: https://galaxyproject.org/use.
Note: The Galaxy EU https://usegalaxy.eu and Galaxy AU https://usegalaxy.au.org servers are both down: EU for scheduled maintenance, should be back soon; AU, not sure of the reason, but expect they will also be back online shortly.