Let me help you find the Plus (+) icon so you can write checks or make deposits successfully.
First off, let's check the "+ New" icon at the top left corner on your QuickBooks Online (QBO) interface. If you happen to locate the icon from there, it means that your QuickBooks is already updated. The latest update of QuickBooks includes the interface of the page.
However, if you can't still see the "+ New" icon from there, let's try logging into your QBO account using a private browser. There are times that the browser is full of frequently-accessed page resources, thus causing websites, like QuickBooks Online (QBO), to act weirdly.