I visited a client that was experiencing an issue with her MacBook Air where she was getting a warning error "No Available Windows" whenever she tried to open an application.The strange thing was when I opened the same application (in this case it was Safari), it opened fine, and this is on the same MacBook.
I then investigated and finally figured out that when she clicked on the app to launch she was pressing the touchpad a bit too hard and long, whenever I opened the app it was a quick but firm tap and the app would open.
I figured out that there is an additional click on the touchpad/trackpad that performs additional functions. You can feel the additional click if you slowly apply pressure to the touchpad, you will get the first click and after continuously applying further pressure there is a second click.
After some research on Google, I found out that this feature is called "Force Click" by Apple and performs various other functions depending on what app is opened.