Congrats on making your first post here in the Community. Let's try a few basic troubleshooting steps to help you see the calendar field and allow you to edit the line items in an invoice.
Using a private browser or incognito window will allow QuickBooks to load as a fresh new page. This will help eliminate any errors or glitches that are occurring in your QuickBooks Online (QBO) account. Here's how depending on your session:
Google Chrome: Ctrl + Shift + N
Internet Explorer/Firefox: Ctrl+ Shift + P
Safari: Command + Shift + N
If this works, go back to your regular session and clear your browser's history. If it doesn't, please provide me with some screenshots or more details on this matter.
Furthermore, here are a few articles that can help you out now and in the future: