Hi Rutvik,
You can check on below points
- On serial item tracking dimensions screen(same in your screen capture) check if your item is available in "Related items" menu. If not you need to assign respective tracking dimensions product.
- I hope you have sync the data to channel properly. To confirm, if you have access to channel database check if the product available in [crt].SERIALIZEDITEMSVIEW.
- If you don't have database access use fiddler to check highlighted property marked as true as below

After all above check point, still if you are unable to get the pop up to enter serial number, I will suggest to compare your product configurations with 82000 product in usrt data.
I hope this will help you.