This message means Microsoft Outlook is already open and running in the background or minimized, and you’re trying to relaunch it. You can only find this and close the Backgroud process complately anytime.
Steps to Fix Outlook is already running. Please close the other window to continue.
Step #1: Click on the Start Menu and Search for “Task Manager“. (OR Use Search Bar)

Step 2: Open Task Manager and search for the Outlook app under the “Processes Tab” or a related background process running. I recommend pressing the “O” key on the keyboard to jump to the Outlook app process.

Step #3: Select Process and click “End Task” in the same window. The Process will now forcefully close and turn off permanently.
Step #4 Now, the Outlook App is ready to open, without leaving the old Process in the background on your Windows PC OR Laptop.

In any case, it doesn’t work, and your Computer freezes. Then I recommend restarting your Windows Laptop or PC using the Direct Power button by pressing and holding down. Please wait for a few seconds and turn it on again.