Hi! I had that trouble too, I contact Roblox support and got the answer, here it is
"To be eligible for a Sponsored sort feature, the experience must adhere to our safety standards and community guidelines and cannot be newly created. If your experience has just been created, please try using the Sponsored sort feature in the next 7-10 days to see if your experience is eligible.
If after that time you are still unable to purchase a space in the Sponsored sort, you may need to wait longer and need to ensure that the experience adheres to the safety standards and community guidelines which can be found here (https://en.help.roblox.com/hc/en-us/articles/203313410)."
So we have to wait 7-10 days now, that’s it