If you are using Quickbooks Online, the undo reconciliation option is reserved for your accountant (I don't know why).   However the good news is you can be your own accountant.   

Click on the gear then "Manage Users" then click on the "Accounting Firms" tab.   Invite yourself to be your own accountant.  (There is no extra fee for this).  You'll get an activation link that will allow you to sign up as an accountant.  Then you can log into and it will then take you to the Quickbooks accountant page. From there you'll see your main account, and will have the ability to undo the reconciliation.  

Tip: when you are inviting yourself to be your own accountant, use a separate email address and create a separate account for the accountant user.    The Accountant view and regular view of QBO is largely the same and 95% of the functions can be performed in either version, but there are a few subtle differences that do not work in the accountant view.  (like the ability to print your billing statement for your QBO subscription).  If you use the same email then you'll end up always logging into the accountant view.  If you keep the accounts separate, then you'll be able to log into either QBO version of the site when needed.   Learned this the hard way.

Good Luck!