Tested on Windows 10.

Steps to reproduce the error:
1. Open BOINC Manager
2. On the menu bar, go to View > Advanced View
3. On the menu bar, go to Options > Exclusive applications
4. Under the "Suspend processor and network usage when these appliications are running" section, click Add
5. Choose a executable of a UWP/Windows app, such as ABC.exe (Where is the executable of a UWP/Windows app? Open the app, and open Task Manager. Within Task Manager, click More details at the lower left, and go to Details tab. Look for the process name of the app, right click on it, and click Open file location. Then you get the path of the executable.)
6. BOINC tells that 'ABC.exe' is not an executable application.

However, I found a workround to add a UWP/Windows app:
1. Add a random Win32 app as an exclusive application, such as EFG.exe
2. Go to C:\ProgramData\BOINC, right click on cc_config.xml, and click edit.
3. Look for "<exclusive_app>EFG.exe</exclusive_app>". Replace EFG.exe with ABC.exe
4. Close and save cc_config.xml
5. Exit BOINC Manager (right click on the BOINC icon on the taskbar > click Exit) and reopen it
6. Now the UWP/Windows app has finally been added to the exclusion application list

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As far as I can research, Universal Windows Platform applications come in a container that has a non-executable extension, like .appx and .appxbundle
That's why BOINC Manager will tell you that this isn't an executable. (Ref: Packaging UWP apps)

Since the manager cannot look inside such a container, it cannot determine whether there are executables available.
A workaround is, to check in Task Manager what the exact executable's name is when the application is running and add that (like you do manually) to cc_config.xml

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