Hi there,
I appreciate you taking the time to reach out to us here in the Community. I'll be happy to shed more light on why you're not seeing your customer list after clicking on the Sales tab.
The reason is that QuickBooks has made it easier for you to access your customer list by click on Leads & Customers that's just below the Sales tab. I encourage you to check on your end to make sure you're also seeing this new tab. In case you're not seeing it, I recommend accessing your QuickBooks account using a private/incognito window.
Feel free to keep me posted on how you make out. I'll be here in case you need additional support. Otherwise, I wish you a great rest of the week!