Can I send an offer only to people who have not ordered for a month?

Yes. Open Marketing → Campaigns, press New campaign, and choose the audience Haven't ordered in a while. Write the message, pick the channel and send; it goes out inside the daytime window under your weekly cap. If you would rather not do this by hand each month, the win-back ladder and the Quiet regulars nudge under Marketing do the same job on their own, one step at a time, and never both fire on the same day. Judge the result on your Dashboard by the orders that came back, not by how many messages went out.