"If you have questions, call me personally."
Translation:Falls du Fragen hast, ruf mich persönlich an.
du and ihr are second-person pronouns, and German has a proper imperative (command form) for the second person -- this doesn't include the pronoun, as in English ("call me" not "you call me" or "call you me").
Sie works grammatically like a third-person pronoun (like "they"), and German doesn't have a third-person imperative. So what it used for commands addressed to Sie is the subjunctive, and that doesn't allow you to omit the pronoun.
If you want to be very literal, then rufen Sie mich an! is something like "may They call me!" (using "may" to represent the subjunctive mood in this example).