The ability to undo a reconciliation is only available in QuickBooks Online Accountant (QBOA) version. To make sure the Undo option is available, verify with the Master Admin if your access is for Accountant, and you're using the correct email address.
Here's how an accountant's version looks like:
To check the user type, follow the steps below:
Select the Gear icon.
Click Manage Users.
Go to the Accounting firms tab.
If you're seeing the same view of the screenshot, then I suggest performing some basic troubleshooting steps to help isolate and fix the issue. To start, sign in to your account through an incognito window. It'll allow you to access QuickBooks without using cookies.
Here are the keyboard shortcuts to open a private browser:
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox: press Ctrl + Shift + P
Microsoft Edge: press Ctrl + Shift + P
Safari: press Command + Shift +N
If it works, I suggest you proceed with clearing the cache. There are times too much data can accumulate over time and cause issues when using QuickBooks.