Update : This problem came from my provider. I changed the server and it worked again. Thanks.

Filezille does not render things correctly at the moment, try using another SFTP client such as WinSCP if on Windows

@PacksGamingHD said in An error was encountered when trying to retrieve this file for editing. [HTTP\1.1 500]: ok so i found the issue: the file was 0.00 bytes and when that happens puffer panel won't let you modify that file so you have to do it from filezilla and add some text to modify it in puffer panel Or delete the file through puffer and then remake it with the contents

So what Puffrfish also said before - you can not add a plugin. But when youy create a server with a exist server type and then go with SFTP to the server files and replace the server.jar with a spigot variant it works. Tck13