In most cases, the Fly the Line app should provide in-app messages as to why a flight cannot be dispatched. The most common causes of a flight not being dispatched are: 

Simulator not detected: This indicates that Fly the Line is not able to detect the simulator. This may be caused by the simulator not being launched, or it can be caused by FSUIPC and XPUIPC not being installed on your PC. Further information on the FSUIPC and XPUIPC requirements can be found in the following FAQ: https://support.justflight.com/a/solutions/articles/17000147409

Time and Date: Fly the Line will automatically set the correct time and date in the simulator based on your schedule. Fly the Line may not be able to set the correct time if the "Track real-world date & time" is enabled in X-Plane 11 and X-Plane 12, we therefore recommend that this option is disabled.  

Incorrect airport location: You must load a flight at the same airport as your departure airport in your schedule. Fly the Line will detect if you have loaded in at the correct airport, and if not, it will generate a message prompting you to load a flight at the correct airport. This message may also be generated if FSUIPC and XPUIPC are not installed on your PC. Further information on the FSUIPC and XPUIPC requirements can be found in the following FAQ: https://support.justflight.com/a/solutions/articles/17000147409 

Unable to Connect to Simbrief: In order for Fly the Line to connect to your Simbrief account, your Simbrief Pilot ID must be entered on the Settings page of the Fly the Line app.