I made a script called - SPEAK assigned to a button

as we know by pressing esc on keyboard we can pause a script

but i want to stop the above script  called SPEAK using 

Code ( Get(TriggerKeystroke) ) = 9 that is equal to TAB key HELP me please

I don't think that's possible. The speech is performed by the OS and no user actions are registered by FMP while it's in progress. Also, a triggered script has no option to pause, exit or halt a running script.

You could make a loop that loops over every word, speaks that word and checks for a keystroke after each spoken word? It might cause an unnatural stutter though.

You could use a text-to-speech web service like the one I used for the 2018 Devcon (https://www.soliantconsulting.com/blog/filemaker-devcon-2018-billy-bass-1/) and have it produce an mp3 file.  Once you have it on a container you can use the AV player functionality of the container to start and stop it.