Droppy requires you to setup a cron job, the cron job will check if uploads are expired and removes the files from your server.
To setup the cron job you'll have to create a job that requests the url https://yourdomain.com/cron every 5 minutes. Setting up the cron job may be a bit different depending on which hosting panel you're using:
Cpanel has a section called "Cron jobs" under the category "Advanced".
Then on the page setup a new cron job like below:
As for the command, use something like:
wget -q -O /dev/null "https://yourdomain.com/cron"
In plesk cron jobs are called "Tasks". Schedule a new task and select the option "Fetch a URL", then enter the URL https://yourdomain.com/cron and set it to run every 5 minutes.
If you're using a linux server without any hosting panel then use the crontab command to setup a cron, more info can be found over here https://www.adminschoice.com/crontab-quick-reference
If you're using another hosting panel then the one mentioned above then please contact your hosting provider and ask them to help you setup a cron that requests a URL every 5 minutes.