Our Guidelines for Reviewers set out that you can review a business if you've had a recent, genuine experience with them. It could be:
You bought something
You ordered something (even if the order was cancelled!)
You had a phone conversation, email correspondence, or online chat
You visited a store
You otherwise used the business's products or services
Top tip: Save some documentation that shows you've had an experience (like a receipt or a screenshot of your online chat with customer service), just in case there's a question about your review after you've posted it.