Are you finding qgis_app.dll just press download button to get your qgis_app.dll file.
If some software in your windows OS wants qgis_app.dll your operating system will ask for DLL file. When that DLL file is not available you will get this error.
If you have moved your file or changed the name of dll file you will get this error.
You will mostly see there errors when you dll file is missing:
If you need qgis_app.dll Download it right now and install it into your system to fix the error as soon as possible. just Install qgis_app.dll and fix your error.