The "EXPECTATION_FAILED" error in User Account Link will be caused by the following reasons.
Check request parameter setting as below:
1. Wrong scope
Check if a correct scope is set.
It is often the case that an unauthorized scope is set, or that a scope is misspelled.
2. Wrong redirectUrl
Check if the redirectUrl of domain is what has been registered/ applied for the call back domain (i.e., redirect domain after user authorization process).
If you use PayPay for Developers, you can check/ set the setting from the call back domain in the Setting under the development environment.