It might be that a webpage issue is preventing you from a creating a bill. In this case, let's start by accessing your QuickBooks Online (QBO) account through an incognito window.
From there, try creating a bill.
To open an incognito window, you can refer to these shortcut keys:
Press Ctrl + Shift + P for Internet Explorer or Firefox.
Press Ctrl + Shift + N for Chrome.
Hold down Command + Shift + N for Safari.
If you're able to create a bill, then you'll have to clear the cache of your regular browser.