Coin Slot Sensor Using AttachedInterrupt
Again, it makes sense to just use an attachInterrupt capable pin; but if can't for some reason:
Library install
Applying what I suggested in the answer on your other question to the code you've provided here, means installing the mentioned library:

And searching for and installing the library from the manager:

Code changes
Including the header
Patterned after documentation and an example for the library, your code would need to include the header for the library:
#include &quot;PinChangeInterrupt.h&quot;

Replacing attachInterrupt
Your attachInterrupt call would need to change to attachPinChangeInterrupt; likewise with the function that maps the pin to the interrupt number:
attachPinChangeInterrupt(digitalPinToPinChangeInterrupt(9), coinInterrupt, FALLING);


That is about all there is to it.  On the UNO/Nano or broadly ATMega328P-based Arduinos, only pins 2 and 3 work with attachInterrupt.  attachPinChangeInterrupt will work with most pins of most AVR-based Arduinos; any pins that have an PCINTn signal routed to them.  These are not as desirable to use, for performance reasons that you don't need to worry about for push buttons or your coin detector, and for easy-of-use reasons that are taken care of for you by this library.