New task not visible

I have completed all the following quest: A troubling Situation Jump-Start Research A Mythical Discovery A Ripple in Time A Thousand-Year Slumbler Let's go, Meltan

After all this task completed, I could not find any new task i.e Pokemon GO World Tourism Day Research Task. Is there any specific reason for that?

Solution 1:

From the "News" item in the Pokemon Go app:

Specially themed Field Research tasks will be available for a limited time during this event.

The world tourism day research tasks are not special research tasks, which show up in the Special Research menu (along with the ones you've named); they are special field research tasks, and can be obtained by spinning PokeStops (assuming you have an open slot for field research).