The reason is that cmd.exe
Being a terminal window relies on input and system Environment variables, in order to perform what you request it to do. it doesn’t know the location of everything and it also does not know when to distinguish between commands or executable names which are separated by whitespace like and or commands with whitespace as switch variables.
First we make sure, is the executable actually installed? If yes, continue with the rest, if not, install it first.
If you have any executable which you are attempting to run from then you need to tell where this file is located. There are two ways of doing the file.
1.specify the full path to the file.
C:\ Directory Path\A.exe
2.Add the location of the file to your environment Variables.
Control Panel-> System-> Advanced System Settings->Environment Variables
Save the changes and exit. You need to make sure that any windows you had open are then closed and re-opened to allow it to re-import the environment variables. Now you should be able to run A.exe from any path, within as the environment is aware of the path to it.
Also, you can refer to the page