Select the Scratch Cat sprite and add a wait block:

when flag clicked
go to x:(200) y:(-150) 
show
repeat (20) // try different numbers
move (5) steps 
next costume 
+ wait (1) seconds
end
hide

Test: Click on the green flag. The Scratch Cat will walk too slowly and miss the bus!

You will want delays of less than one second. 0.5 is half a second, 0.25 is a quarter of a second, and 0.1 is a tenth of a second.

Change the delay in the wait block:

wait (0.2) seconds // try 0.1, 0.5, 0.05

Test: Click on the green flag, and the Scratch Cat will walk faster. Choose the delay that you like the most.

If you want the Scratch Cat to miss the bus, remove the hide block from your code so that the Scratch Cat stays on the Stage:

when flag clicked
go to x:(200) y:(-150) 
show
repeat (20) 
move (5) steps 
next costume
wait (0.5) seconds 
end
-hide

If you want the Scratch Cat to catch the bus, make the bus wait longer before it leaves:

when flag clicked 
+wait [4] seconds // change from 4 to 6
glide [2] secs to x: [320] y: [-100] // right-hand side of the Stage
hide

You will need to put the hide block back in the Scratch Cat sprite's code if you have removed it and want the Scratch Cat to successfully catch the bus.

Make changes until you get the animation to work the way that you want it to.