i dont know how to disable keep inventory on death for staff members while in the server itself keepinventory is off

for some reason it cant find the whole permission in group manager within the owner group

This permission node doesn't match any node.
But might match node: *

You shouldn't give anyone *, as this causes permission issues like this. Give users what they need. Also, not sure if GroupManager can even negate permissions. Maybe check out LuckPerms?

is that easier than groupmanager?

Yes, and supported

They probably have one or both:
essentials.keepxp
essentials.keepinv

Negate these or don't grant them to begin with