How Can I Homebrew a Rogue-Assassin? 

Is there something wrong with the Assassin subclass in the Players' Handbook?

yeah its not in the subclasses for Rogue

Have you bought the players handbook on Dndbeyond?

Collections>create subclass in the homebrew list

You will basically need access to a complete copy of the subclass to do it, but it is possible if you'd rather not buy the same content multiple times.  Just remember not to publish it since you didn't create it originally.

I understand that but I don't understand how to make the abilities of the subclass show up on the character sheet.

Most likely problem is make sure homebrew is enabled on step 0 of character creation.

ok thanks that helps a lot