What 'a mark of success' means?

It's much better if you provide the example sentences you have in mind.

For example from the book:
"But a key factor seems to that today, young people define living alone as a positive way, as a mark of success"

For me it sounds something like 'an ingredient of a recipe of success'

Sorry, mistake
"But a key factor seems to BE that today, young people define living alone as a positive way, as a mark of success"

But from another point 'mark of success' sound like 'signs of success' for superstitious people, for example:
"She saw a beautiful horse crossing the road and treated it like a mark of success"

I don't know what book you're using, but it appears to have mistakes in it!

"mark of success" is natural and common in English. Use it!
I hope this answers your question.

teechr"mark of success" is natural and common in English. Use it!

So what does it mean?

Petr SadfSo what does it mean?

Sign or symbol of success.

Ahh ok thanks a lot

You're welcome.