The first backpacks plugin that I brought, had an error and it was specifically that when the data folder did not exist but the plugin was enabled, when the server was reloaded, the connected player did not have any backpack inventory access and an error was displayed. This was resolved by adding a check when starting the plugin, that if the number of players is greater than or equal to one, then for each connected player it does the same as when a player joins to the server, adding a space in the backpacks.yml Unfortunately, if the server version is not 1.20, the item that will be obtained will be the chest. For the rest, if it is 1.20 it can be completely customized. If not, only some things could be.