It's good to see you here in the Community, tshirtguy.
To ensure you can convert the items from non-inventory to inventory, you'll need to make sure that the As of date comes after the most recent transaction. This way, you can convert the items and save the changes without re-entering them on the spreadsheet.
If you're still unable to convert them, let's go through some basic troubleshooting steps starting with opening a private browser. This helps us determine if the error is caused by the browser cache or not.
Use these keyboard shortcuts depending on the browser you're using:
Google Chrome: press Ctrl + Shift + N.
Mozilla Firefox: press Ctrl + Shift + P.
Safari: press Command + Shift + N.
If you're able to convert the items via the private window, go back to the main browser and clear its cache to start with a clean slate.
Additionally, you can visit this link if you need help in managing your items: Set up and track your inventory in QuickBooks Online. On the same link, you'll find what report to pull up to see your best sellers.
Ping me in if you have additional questions about QuickBooks items. It's my priority that all of your concerns be addressed.