What follows is offered as a public service announcement and reminder regarding what to expect under the current delivery paradigm for Feature Updates that’s been in place since Version 1903.
It could be months after the initial release date before the latest Feature Update to Windows 10 is offered to you in the Windows Update Pane of Update & Security Settings. If it's not there, your machine is not ready. Just check back at a later time and keep checking off and on until it does appear.
Note well the, "When you're ready for the update," part of the final sentence in the notice. You are not being ordered to download and install the Feature Update right now, but merely being offered the option when it suits you. Activating the Download and install link does just that. Fully automatic updates are, effectively, a thing of the past. So, watch your Windows Update Pane for this notice. Remember if at least one month from the first day of release has not already passed if you activate it you are consenting to participate in the "shake out cruise" that occurs in the early weeks of each and every feature update. There are also times where new issues reveal themselves during the rollout period, and your machine can and will go from having the update is available notice to the "not quite ready" notice if your hardware could potentially be affected by those issues. It will eventually go back to update available status once those issues have been addressed.
It is best practice not to force a Feature Update ahead of the time that Microsoft presents it for download and installation in the Windows Update Pane of Update & Security Settings. And if you do not need anything that's in a given feature update, it's a good idea not to activate the Download and install link until at least one month or two has passed from the initial release date.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore