QuickBooks Online lets you downgrade your current plan. However, "downgrade stoppers" can cause a hiccup upon downgrading or canceling your subscription due to some features that aren't available to other versions.
These downgrade stoppers happen if you move to a plan that doesn't have any of the features below:
Recurring transaction templates
You need to turn off these features or limit users so you'll be able to move to another plan. Feel free to read through this article for more details: Downgrade your QuickBooks Online plan.
On the other hand, clearing the cache and cookies on your browser will help restore the functionality of the Cancel button. The cache is stored to speed up browser loading times. However, too much data can accumulate, impact processes, and caused unexpected behavior. As an alternative, make sure to use other supported browsers to get the best out of your QuickBooks experience.
If you're getting the same results, I recommend contacting our QuickBooks Online Technical Team. They can do-screen sharing with you and verify what's stopping you from downgrading or canceling your subscription.
Here's how to get in touch with them:
Sign in to your QuickBooks Online account.
Click Help (?) at the top right.
Select ContactUs to connect with a live support agent.
Enter a brief discussion of your concern, then click Let's talk.
Choose a way on how to get in touch with them.
Let me know how everything turns out by leaving a comment below. I'm always around to help ensure your success.