Hello Discipuli!
This problem can be from one of the plugins you've installed, especially permissions plugin. One of the way I have tested the /seed command is through my vanilla Minehut server where I'm able to do that without any issues so this may involve one of your plugins affecting the permissions. One of the way I will suggest you to diagnose this issue is to go onto whichever permission plugin you used and give yourself full operator permission. To do that, refer to the following:
LuckPerms: /lp user <your username> permission set luckperms.* true
PermissionsEX: /pex user <username> add *
Group Manager: If you are op you should get all permissions no matter what group you are in.
IF your permissions plugin is Group Manager, I will suggest you to uninstall that plugin and change a permissions plugin as other ones right now are more reliable than Group Manager. I personally suggest LuckPerms but PermissionsEX are also a pretty good choice.
If those does not work, I will suggest you to uninstall each plugin one by one to see which plugin is affecting it. (This may sound inefficient but trust me, you'll be glad once you can do the /seed command.)
For a faster option, you can just disable all of your permissions plugin and don't use it. But I know that you would want to keep it so maybe try diagnosing it with the methods above. Good luck.