This might be a browser-related error. Let's begin by accessing your account in a new incognito or private window of your browser. This way, we can check if this issue is related to your current browser or not. Let me guide you how:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once completed, you can log back into your account and check if the reports are available. If it's already there, you can clear your browser's cache to optimize it. Also, your browser can start from scratch and improve your QuickBooks Online browsing experience.