Just to report empty outputs and errors when trying to plot the Velvet LastGraph created using the DeBruijn Graph tutorial (Hands-on: De Bruijn Graph Assembly / De Bruijn Graph Assembly / Assembly). This happened to me and 30 students working on their own laptops.
Welcome, @Ferran_Palero
Sorry that happened and thanks for reporting the problem! I’ll check that tutorial today and see if that can still be reproduced. If you want to share back a history link with an example, that might help too but I’ll ask if really needed. More soon! 
I see the problem. The most current version of Bandage doesn’t work with the older file format output by velvetg.
Next time, please try this: use the Versions menu to navigate to the earlier tool version and use that instead. How to → FAQ: Changing the tool version
I’ll contact the GTN and we can probably add a note to the tutorial warning about the need to do this, and it will link in the FAQ I shared too. Update debruijn-graph-assembly tuto: Bandage tool version · Issue #5544 · galaxyproject/training-material · GitHub
Thanks for letting us know! 