I'm trying to cancel my Dropbox subscription, but there is no cancel button in my subscription options. Dropbox says they cannot help me, since the sub is in Apple subscriptions.

If the option to cancel a subscription is missing, see the following excerpt from Apple Support article If you want to cancel a subscription from Apple - Apple Support

You might need to scroll down to find the Cancel Subscription button. If there is no Cancel button or you see an expiration message in red text, the subscription is already canceled.

Also, a subscription that is already cancelled will typically have wording in fine print similar to "Expires on [date]" rather than "Renews on [date].”

This is primarily a user-to-user support forum; other Apple users like you.

If the above didn't help, see: Subscriptions and Billing - Official Apple Support

If there is no Cancel button, why do you think it isn't already cancelled?

Also, have you tried contacting Apple Support?

See the following from my previous post: If the above didn't help, see: Subscriptions and Billing - Official Apple Support

or see Contact - Official Apple Support