Wait, arbitrarily. (Honest, straight, direct, plain, true answer. Seriously, it might/does randomly appear. Wait. Reload. Wait.)
I say Wait specifically, because every ~few years the "Search History" Download/Takeout shows on myactivity.google.com arbitrarily. I am not joking, and your question sounds like you thought you were being fooled but I understand, you're confused why and it's understandable.
(I do not know why and when the timing, Google does not feel it is important for users to know when their Search History archives will be there. For some reason not specified in their sparse Help sections, Google does not feel required to report answers to users basic questions, like these about our primary data relationship with Google, searching.)
Blissfully written, "How to download your Google data - Google Account Help" (https://web.archive.org/web/20211212222758/https://support.google.com/accounts/answer/3024190#) states "Important: If actions are deemed risky, to protect your account, they may be delayed or unavailable." signaling ignorantly/pompously in an elitist it's-for-information-security-voice that there might be legal reasons/rationales for these coded secret affairs being justified as not worth writing a Notification/Error Message.
In my experience, it has shown before and will show again, depending on Google's deep problems. (I am not being funny, it sounds funny, but given there is not any law/standardization nor spokesperson/representative responsible for these questions, you have Google being/sounding funny.) I know it sounds like Google is playing with you, but it's true.
You and I are not alone, us not being given clear English error messages is seemingly standard legal practice for how these companies avoid responsibility and play the issues as a "bug" er "feature" (which of course is not described, in any official public writing, about search data).
Err, maybe there is profit in harming users by not letting us find our own data, yet somehow having it leaked and adjudicated at the same time. I keep reading about my data being leaked/hacked WHILE I can not access my own data.
Aside from that, if you have law enforcement connections, I've read they're able to access full data dumps of search history. Legitimately, a cop can read in-the-blink-of-an-eye your Search History fully, but apparently Users do not have the Googod-given right to your life's data.
Finally, I appreciate the question author's integrity going out on a limb to ask the obvious missing question, you would think that the trillion-dollar SEARCH engine company would let you download your own ("uploaded") SEARCH history.
For me, it did not show in 2019, then it showed last year in 2020, but not this year 2021, so I am, praying to Goog-d and, well, waiting.