I get this error on a ton of players wile performing any command related to ban manager.
"Player not found, are you sure they exist?"

Im using the latest version altho not the dev one.

Commands usually require a player to have joined the server at least once since BanManager has been installed. Otherwise it'll perform a lookup to Mojang to grab their UUID, which is rate limited. In this case when it hits the rate limit it'll simply fail the lookup and say not found.