Cannot add site after I upgarde to wordpress business plan and have new domain

At first, I use wordpress free plan with domain name "rocnationaltest.wordpress.com". The addon worked.

But after I upgrade to wordpress business plan and have new domain, I cannot upload google docs. meanwhile, I cannot add my new domain "taiwan-exam.com"

The error massage is " Token response not valid JSON: SyntaxError: Empty JSON strinf".

Start by reproducing the Google Docs upload and domain-addition flow with a WordPress Business plan and the new domain taiwan-exam.com. Inspect the token response around the reported empty JSON error; done means the domain can be added and Google Docs uploads work again.

Written by the indexing model from the issue text.