Let me share some information and fix your concern with importing inventories into QuickBooks Online (QBO).
I’ve replicated this by importing my excel file to my QBO company. Then, found out that the import button is available on the mapping window. However, I noticed that the whole page is not showing right after proceeding to this page. With this, we can zoom out the page to do the trick:
Click the ellipsis icon ⋮ in your browser located at the top right corner.
Minimize the text page by pressing the Negative (-) sign beside the Zoom option.
Stop until the whole page is showing, and start importing your inventories.
If the issue persists, we can launch the website in a private or incognito browser. Your browsing history won't be saved in this mode, which also aids in isolating any website problems.
Here are the keyboard shortcuts:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Shift + N
If it works, you should clear the browser's cache. This way, the system can start fresh. Also, different supported browsers can be used as an alternative if you have the same issue when using a private browser.