Are you talking about Google Play Store? Are you getting an error message? Does this happen on both wi-fi and mobile data?
Try to clean your cache as well as app data of Google Play store. and restart your device and just open play store and start downloading app.
If you have any update recently installed for your google play store uninstall it and restart your phone and try it again it will work. Also check for Restricted Background data.
Please keep me updated, thank you!