"You should come alone."
Translation:Měla bys přijít sama.
Yes, you can skip the personal pronoun in most Czech sentences. We accept with or without as it is optional in Czech. Do not rely on what the system underscores as a mistake though. It gets confused if your sentence is really off and just underlines the first wrong word that is not wrong per se. We need to see your entire sentence to be able to tell you if and what went wrong.