I was not given the reward of the stash spot, or the portrait upon the completion of this seasonal quest category of Conqueror.
I completed the 8 objectives (even the game shows so) and after, received nothing. So I immediately restarted and still no reward. It even shows that I have not “unlocked” the rewards, but it shows to be completed.
No I do not have the max amount of stash tabs either.
Not a bug. Complete the previous Chapters too.
Not a bug. You can’t just complete one chapter. You have to complete all the previous chapters first.
This is why we can’t have nice things. Blizzard finally allows us to look ahead the the future chapters for our own benefit, and people like you try to cheat and only complete specific chapters.
I know right. Like you think people would know that prior chapters are required. Its almost like some just need to be hand held.
I mean “yes” they could of added an objective to each chapter that states the prior one has to be complete as well to “prevent” confusion. But really why does anything think this would be different then the prior 16 seasons.
That is exactly the point. 
And what is annoying is some players can’t even take the time to read the current Season Blog so they know exactly what is going on in the current Season. Add to that, if something doesn’t work the way they expect it to, they post it as a game bug.
From the Season 17 Blog - Cosmetic Rewards
Players who complete all nine chapters of the Season Journey will be rewarded with a new cosmetic item —and for Season 17, that means the Galactic Wings could be yours!
Source: Season 17: The Season of Nightmares is Now Live - Diablo III
From the Season 17 Blog - Season Journey Rewards
If you’ve been diligently slaying demons for the past few Seasons and reached Conqueror in the Season Journey each time, you’ve surely accrued a few extra stash tabs. You’ll earn one additional tab each Season by finishing the Conqueror tier, up to a maximum of five
Source: Season 17: The Season of Nightmares is Now Live - Diablo III
Same thing just happened to me. I double checked and I have completed all previous requirements. It’s the only reason I did the season.
Did you already have 13 tabs prior to this season? If you did, then you had already earned the maximum amount. The number of tabs is capped at 13, this includes the total 5 earned through seasons.
Don’t even try to say you didn’t know since Blizzard posts this information in every season blog.
i am having this same problem. i only have 9 tabs. i completed everything even got my pet cat and still no extra tab in stash like promised. what am i missing?
only reason i knew this was a thing to get an extra tab is because my friend said they had 13. Do i have to wait until next season to get 10? and the following to get 11? Is this how it works?
A player answered here : https://us.forums.blizzard.com/en/d3/t/quick-stash-tabs-question/16192/17
thankyou. it makes complete sense to me now. i appreciate the clarification.