I have tried both using a separate gmail account, and opening sheets in an incognito mode window and on different computers but I still keep getting an error that I cannot configure my merge with multiple accounts. Please advise.
Sorry for the inconvenience.
It looks like Google has made some changes to the Gmail service at their end that has broken all addons that connect to Gmail.
You can track the issue here https://issuetracker.google.com/issues/160586526
As soon as Google fixes the bug, the add-on will resume working.
We are closely tracking this and will update you as soon as we hear from them.