Discord doesn’t allow bots to purge (bulk delete) messages that are older than 14 days old.
You may or may not have seen other bots that will delete messages older than 14 days one at a time, but Dyno does not do that and it is unlikely that this will change unless and until Discord makes the feature available to us.
If you would like to completely purge a channel, you can right click on the channel and select “Clone Channel”. This will create a new channel with the exact same settings as the old. You can then delete or archive the old channel.