The warning is normal. Do /plugins to ensure you have the appropriate plugins enabled. Also be sure to OP yourself.. How to OP yourself
File Manager > Press on the console tab > type in 'op username', no slashes.
If you are OPPED and the command does not exist, it is likely also red in /plugins
Ensure the plugins are compatible and the correct version for 1.16.5 specifically, also.
To give all players the permission, and I recommend using Luckperms, this command will NOT work with PEX: you do /lp group default permission set essentials.spawn true
Overall, the permission node is essentials.spawn, regardless of plugin. Just look up how to utilize PEX and that permission node will work.