Hello Shshzaib,
If you haven't already, we would suggest that you power cycle the iPhone using these instructions: Restart your iPhone - Apple Support While simple, this can refresh the iOS, apps and connections.
Once the iPhone restarts, you may want to try a different Wi-Fi network to see if that helps.
We also recommend working through these steps: If you can’t download or update apps on your iPhone or iPad - Apple Support
If you still have issues downloading apps, Apple Support would be the next stop. They can be reached here: Get Support
Have a good day.