Hi there @AizazUlHaq !
Thanks for sharing your concern with us here in the Shopify Community.
There are a couple of things I’d like to check with you, to see what might be happening here.
First, I’d check to ensure that you’re using a browser that is supported for use with Shopify.
If it is, I’d then proceed to clear the cache on that browser, as issues like this are often affected by browser data stored locally.
Could you confirm if you’re logged into your Shopify store already on that same browser?
If you’re still having any issues with adding an app after running these checks, please click on this link and get back to me here with the short URL you are given on that page so we can take a look at some more info on your browser together.
I look forward to checking this out with you!