Thank you for your reply, I have registered a subdomain, I changed it to “demo” subdomain before I sent out the code.
I has test both of demo domain and my registered domain, and the API return the key successfully and produce exactly the same result.
I think the problem is in V2 EQUIP API. I follow the guide (Custom Avatar Creator | Ready Player Me), and only the EQUIP step does not provide the expected result.
Same problem in this post (Trouble equipping assets - #4 by wantr0ba).