I'm here to help ensure you're able to create an invoice in QuickBooks Online, userj-chad-j.
There are times that your default browser is filled with data and temporary files from previously visited websites. With this said, I recommend using a private browser. Doing so helps isolate the issue.
If not, you need to check the access given to you by the Company or Master admin. You'll need to have permissions to access the areas of Customers & Sales to be able to create invoices.