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 are using PayPay for developer, you can check (set) your settings from the callback domain from PayPay for developer in the test environment.