The most important component of any PVP plugin is to fight against each other. Often it happens that different settings prevent this. Therefore, before you create a ticket, check the following:
PVP is disabled in the server settings
It is possible to disable PVP for the entire server in the server.properties file. Open the file with a text editor and make sure that the pvp=true option is set.
Wrong Multiverse settings
Most servers use the Multiverse plugin to manage their servers. Check in the /plugins/Multiverse-Core/worlds.yml file whether PVP is disabled. The setting can be found under worlds.[Your World].pvp, which should be set to 'true' for active PVP.
WorldGuard flags
WorldGuard is the most popular plugin for managing regions. PVP can also be deactivated there. So, go to the appropriate location where PVP does not work and type the /region info command. Now go through all listed regions and check whether the flag pvp: DENY is set. If this is the case, you can enable PVP with the /region flag [Regionname] pvp allow command.
Other plugin
If the suggested solutions did not help, disable the server and move all plugins from the plugins folder to another. Now install only the corresponding PVP plugin and start the server. Now test whether PVP works. If it does not work, please create a ticket in the ticket system. If it works, you can reinstall one plugin after another to check which plugin is causing the problem.