oxide.grant group default "InvisibleSleepers.allow"

oxide.grant group default invisiblesleepers.allow

Try it without the capitol letters.

Permission 'invisiblesleepers.allow' doesn't exist

That generally means the plugin is not loaded. Casing of the permission will not matter either, as all permission input for those commands is converted to lowercase. I'd suggest checking your oxide/logs for errors and the 'plugins' command.