I appreciate your efforts in following my colleague's resolution, rain. Let me chime into this thread and offer additional information so you can start tracking your inventory.

To proceed, were trying to select a date for your items before you started tracking your inventories? If so, please note that the mentioned warning will appear if the date on the transaction you want to create comes before the As of date. In this case, we verify the starting value of the item and ensure the dates are set up accordingly. Let me show you how: 

1. Go to Products and services. 
2. Locate the item and hit Edit. 
3. From the Quantity on hand section, click Starting value. 
   
4. Hit Got it when prompted.
5. Review the As of date. 
6. If it's incorrect, we can change it. If it's not, leave it as is. 
7. Select Save and close. 
   

You can visit this article for more info: Set up and track your inventory in QuickBooks Online. 

However, if you're trying to choose a period after you start tracking, I recommend we perform some basic troubleshooting steps to rule out the possibility of a browser issue. 

We can open your account on an incognito window. Doing so prevents the browser history from being saved. Refer to the following keyboard shortcuts based on your browser type:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P
• Safari: Command + Shift + N

If the private browsing session works, clear the browser's cache so the system can restart. This task is also possible in other supported, up-to-date browsers. Clearing the cache will reboot the system, allowing you to work with a clean slate. 

Moreover, you can browse these resources to manage your inventory in QuickBooks Online (QBO): 

• Adjust inventory quantity on hand in QuickBooks Online
• Use reports to see your sales and inventory status

I'm still ready to assist with any product and service-related questions. Stay safe always!