"Your name is Marcus."
Translation:Nomen tibi est Marcus.
That would be acceptable, but bear in mind that "esse/to be" is a copular verb and as such takes a subject complement and not any kind of object. This means that you'd have everything in the nominative on one side of "est".
I've written: "Marcus est nomen tibi". But it is not correct :( I would like to know why? Thanks in advance!