Thanks a million for sharing this video.
I noticed that you are using two Google accounts under the same browser (see below)
Could you try to create a new Chrome profile for the account you use to mail merge? It takes 30 seconds and doesn't require a password, it's just a separate profile in your browser. In your case I suspect that since you are using multiple Google accounts, Google is confused about which account to use, causing this error.
Alternatively, a solution can be open a private window and retry.
Keep us posted!
Thank you for your patience.