If your subscription status is Free, you may have a second account with Spotify. It's worth checking your email inboxes to see if you have any Spotify emails - that can mean that your extra account is registered with that email.
If you don't have another email address, note how you log into the Spotify app usually:
If you use your Facebook details try using the blue "Log in with Facebook button."
If you use your Spotify username try entering your username or email address.
Once you've logged in, try the cancel process again and ensure you reach the final confirmation screen.
Tip: If you subscribed through any other company than Spotify (for example your mobile or broadband company), you need to contact that company to cancel.