Thanks, Maximus

It turned out that selling the capitol (Maccua) was the trick to make my tactic work - Qohaito needs to have Maccua in order to release it as Saba.

I assume it has something to do with the culture makeup of the province? On the wiki it does specifically state that primary culture pops are needed.

Saba is a big nation, so it would be nicer if I could get it released it as a smaller nation instead. So I thought of this strategy: I sold the Province of Saba to Qohaito, then fought a war successfully with Saba, and wanted to Demand Peace with Qohaito, and make "Release Saba" as one of the Peace terms. That way, Saba should be released on a rather small plot of land - ie. just one province. But when I tried to follow this special strategy and conquered Qohaito, the Peace Negotiations don't allow me to ask Qohaito to release any nations, as the screenshot above shows. Why not?

I thought that it might be because Saba's national capitol is located in Maccua, in a more eastward province than the Province of Saba that I gave Qohaito. But on the Imperator Wiki about releasing nations, it doesn't say anything about requiring original capitols to be part of the released nation.
https://imperator.paradoxwikis.com/Warfare#Release_Nation

I played 90 years as Judea in the Invictus Mod, and conquered Saba relatively early in the game. Then I realized that one of the mission tasks, named after Solomon and the Queen of Sheba, is to clientize Saba. This mission task gives you special bonuses at a shrine dedicated to King Solomon. Jericho is a territory where Solomon in real life built a city, so it's a good spot for a shrine for Solomon.

I found that if I start a war with Qohaito, I can Release Saba as part of my peace terms under "Offer Peace." Then I can Clientize the newly-recreated "Saba" nation to meet the Mission Task.