Hi,
Sorry for the inconvenience caused by this issue.
Based on your description, a use can't add Project tab in Teams channel, the message says Project isn't turned on your organization. However, it seems you have tried to turn on Project but issue persists.
(If I misunderstand anything, feel free to correct me)
From the article you shared, it says
It seems actual activation for Project for the web won't happen immediately. You need to see your Message Center for more details.
Have you checked Message Center(from Health>Message center in Microsoft 365 admin center) and found any message about Project activation? If you can't find any message for Project, you may need to wait for some time then check the result.
If you find Project activation message from Message Center but the user still cannot create Project channel, I'd like to confirm information below for further troubleshooting:
1.If the user creates another channel in the team and then try to add Project tab, does same error message appear? What about if he tries a different team?
2.Do other users in your organization have the same issue?
Appreciate your time and effort. Have a nice day!
Best Regards,
Tina