Why can I not send statements on the mobile app? I do pay a subscription

You can only send statements using a web browser. To access your statements, head over to qbo.intuit.com on your computer or mobile browser. The QuickBooks Online mobile app doesn't currently support this feature, and it isn't tied to your subscription plan. 

For step-by-step guidance, you may refer to this article: Create and send customer statements in QuickBooks Online. 

Let me know if you need anything else. I'm always here to help.