Your software
My Mautic version is: 3.1.1
My PHP version is: 7.2
My Database type and version is: MySQL (no idea of the version)
Your problem
My problem is:
Plugins are not showing up when I drop them in the plugin folder. How can I solve this: Thank you 
In the picture what I see in MAutic and what I have in the plugins folder
Have you tried clearing your cache and then clicking install/upgrade plugins?
Check logs for permissions related errors. Typically located in Mauticdirectory/var/logs
Try enabling debug by opening mauticdirectory/app/config/local.php and setting debug to “True”
May have to set permissions manually.
Thank you EJL for your kind reply. In the end I solved it not by cleaning my cache (I tried and it didn’t work) but by reinstalling it.
Probably something wrong with the older version. Thank you
