Seems like it is still not released for Sri Lanka. You can check the dates for all countries here: https://www.sammobile.com/samsung/galaxy-a21s/firmware/#SM-A217M
For Sri Lanka check here: https://www.sammobile.com/samsung/galaxy-a21s/firmware/SM-A217F/SLK/#SM-A217F
Both lists are updated everyday.
Build Number : A217NKSU5CUC7
Edit: I just realized the URLs are not shown in app. Will copy the links here.