Hi,
The plugin seems to be called "User - HikaShop" on your website.
I can also see it hasn't been updated for 7 years.
I suppose you must have disabled it at some point because it broke, because it was not up to date.
So before you enable it you need to update it.
Normally, each time you update HikaShop this plugin is updated together with it automatically.
Since that was apparently not the case, it means that the files of the plugin are probably not writable by PHP.
So first, you need to rename the folder plugins/system/hikashopuser/ via FTP in order to deactivate it.
Then, install the latest version of the HikaShop install package and it will try again to install the latest version of the plugin. So you should get again the folder plugins/system/hikashopuser/ but with the latest version of the files.
Then, you should be able to enable the plugin again and not get that message anymore.
This plugin is really necessary for many things HikaShop needs to do, like the synchronization between the users in Joomla and the users in HikaShop. Without it, if you delete a user in Joomla, the entry for that user in HikaShop won't be deleted for example.