There are ways to get a Ryobi lawn mower battery if it arrived without one.
Check the product description to see if the battery is included. Check your Ryobi or retailer order confirmation email or receipt. The description should state whether the battery was included or sold separately. Additionally, visit the Ryobi website and find the product page for your lawn mower. The product description should also state if the battery is included or sold separately.
Contact the seller to resolve the issue. Tell Ryobi customer service that your lawn mower arrived without a battery. Ask about the discrepancy and see if they can replace the battery or arrange a return/exchange for a complete set. Contact the retailer's customer service if you bought the lawn mower. Explain the situation and ask if they can help you find the missing battery or arrange a return/exchange for a complete set.
If you possess more insights or details, feel free to share them in the comments or file your complaint.

