I have recently started using SS for Valheim and tried adding my Steam64ID (saving and restarting the server), but I too am getting the 'you are not an admin' message. Did you find a resolution to this?
Edit: To anyone experiencing the same problem, my issue is now resolved. After uploading a new world, I now have admin rights. Not sure of this will help everyone, but thought I'd mention it just in case someone can benefit from it.