Go to Settings > Apps & Notifications > Special App Access > Install Unknown Apps .

If you missed the notification, open your Files app, go to the Downloads folder, and tap NDASAT_base.apk .

Only download the file from an official government or institutional website to avoid malware.

Open your mobile browser (like Chrome) and navigate to the official download link.

If you'd like, I can help you or explain how to troubleshoot common installation errors .

How to Install APK Files on your Android Phone (Best Method)