It'd be my pleasure to lend a hand and ensure you'll be able to enter a deposit in QuickBooks Online.
This usually happens if your pop-up windows are blocked. Depending on your browser, you may need to toggle your settings and disable the block pop-up windows option. Let me refer you to this article for the steps on how to enable pop-ups: Why doesn't anything happen when I select a button or link?
QuickBooks Online uses pop-up windows to perform many of its tasks. To know more about this, you can also check this link: Enable pop-up windows.