The trigger "Lead hasn’t made purchases in the last few days" is used to kick off automations and is simple to set up. Before using this trigger, keep in mind:

To set up a trigger like this, you need to:
In the left menu, click on "Automations".

If you already have an automation, just access it. Otherwise, you’ll need to add a new one from the side menu
Click the "Add trigger" button

Go to the lead’s trigger options

Select "Lead hasn't made purchases in the last few days"

Enter the number of days without a purchase for the customer to reach for the trigger to fire

Done! Now you just need to save and activate the automation to use it