"My name is Marcus."
Translation:Nomen mihi est Marcus.
Latin is an SOV language and an acceptable translation should be Nomen mihi Marcus est.
I'm only beginning Learning Latin at university but this is what I have read in my textbooks also
Sum/to be is a bit of a special case. It's the copula and therefore takes a subject complement rather than any kind of object. Think of it as like an equals sign. "Nomen mihi Marcus est" is acceptable, but it has everything in the nominative on one side of "est".
Because you would be leaving out the "is". As in "my name Marcus" instead of "My name is Marcus"