Work that cannot be retrieved by your audience is classified in APA style as a Personal Communication. In your assignment you do still include an in-text citation; however, you do not include this source in your References.
includes emails, text messages, online chats or direct messages, personal interviews, telephone conversations, live speeches, unrecorded classroom lectures, memos, letters, messages from nonarchived discussion groups or online bulletin boards etc. (APA Style, n.d.).
Rather than offer an alternative, T. Williams (personal communication, September 2, 2019) stated that ...
... justice had been served in this case (T. Williams, personal communication, September 2, 2019)
No citation required.