Make sure you have read this article first
If after following the steps in the above article you are still unable to cancel your account, please try the following steps:
Try cancelling it through the website
Go to fitplanapp.com
Click Login from the top navigation menu
Enter your email and password and click Login
Click Cancel Membership
Click OK
Try another email you use or to login with your Facebook account
You might have accidentally created two accounts with two different emails
You might be trying to cancel a free account that doesn't have a paid subscription associated with it