Hi, LAG1@. Thank you for sharing more details with your concern, this will help me narrow down to the cause of this issue.

I can see that you've checked out the previous troubleshooting steps given by my colleagues. Since it's still not working, I'd recommend updating your QuickBooks Desktop to the latest release first.

Here's how:

1. Open QuickBooks Desktop and go to the Help menu.
2. Select Update QuickBooks Desktop.
3. Go to the Update Now tab. Tip: You can select the Reset Update checkbox to clear all previous update downloads.
4. Click Get Updates to start the download.
5. Once done, accept the option to install the new release.

Once done, you can try to check if the Max section of the inventory item is there, if there's still no option to enter the Max value I'd recommend running the QuickBooks Verify and Rebuild tool. This can help identify and repair data damage.

Here's how to Verify your data (original company file):

1. Go to the File menu at the top.
2. Select Utilities, and then choose Verify Data.

If it detects an issue, you can Rebuild your file. Allow me to guide you how:

1. Go back to the File menu.
2. Choose Utilities, and then select Rebuild Data.
3. Select OK, and then follow the on-screen prompts to save a backup.
4. Click OK when you see Rebuild has been completed.

To know more details about the process, please check out this article: Verify and Rebuild Data in QuickBooks Desktop. 

Please don't hesitate to share an update after following the process above. I'd be happy to help you further with any of your concerns. Take care!