Hello there, akumar99,

Thanks for the screenshot. Great job for updating QuickBooks Desktop to the latest release. It's the first step to take in fixing the unexpected behavior in the system.

I'm able to save a to-do item in the system. The screenshot below will show the reminder I've added.

I've found that some customers run into this issue when they have Microsoft SQL Server 2012 installed. Some users have success uninstalling it to fix this. You can check to see if the app installed. If you do, I suggest consulting whoever maintains your computers to determine if removing the server is the right option.

If that's not the case, let's use the sample QuickBooks file and create a reminder from there. This is to test if the issue is with the data on your company file.

1. From the File menu, select New Company.
2. Choose Start Setup and fill out the required fields.
   
3. Click the reminder icon, then select the + (plus) sign.
   
4. Enter the necessary information to complete the task.
   
5. Click OK.
   

When that works, the data on the original file may be damaged. You need to run the verify and rebuild data to fix this.

You can also refer to the System requirements for QuickBooks Desktop 2018 and Enterprise Solutions 18.0 for more details. It will show you what operating system, hardware, and software needed to run the program.

Let me know how everything turns out by leaving a comment below. I'm always around to help you out.