As it says in the docs (https://discordapp.com/developers/docs/resources/guild#create-guild-ban) the ban reason and delete-message-days need to be provided as "query string params" and not json
How it should look: api/v6/guilds/{guild.id}/bans/{user.id}?delete-message-days=1&reason=banreason
https://github.com/PsychoTea/Oxide.Ext.Discord/blob/b26b6765003898c683b97c76bf78db73c644f5c2/Oxide.Ext.Discord/DiscordObjects/Guild.cs#L210
As it says in the docs (https://discordapp.com/developers/docs/resources/guild#create-guild-ban) the ban reason and delete-message-days need to be provided as "query string params" and not json How it should look: api/v6/guilds/{guild.id}/bans/{user.id}?delete-message-days=1&reason=banreason