You’ve mentioned an error, can you tell me more about it?
The process of merging accounts isn’t reversible as of the moment. It is permanent and cannot be undone at a later point.
I can see how this option helpful to you and other users. I'll be sending this feedback personally to our management team so they could come up with the best action to improved QuickBooks. For now, you can visit our blog site where all product updates such as features, apps and etc. are posted.
You can read this article to learn more about merging accounts in QuickBooks. It also includes instructions on how to combine customers and vendors.