APK files are the format used by Android to distribute and install applications. If you're having trouble installing an APK file, there are a few steps you can take to troubleshoot the issue. First, make sure you have enough storage space on your device. If you don't, try freeing up some space by deleting unnecessary files.
You may also need to use a third-party split installer to install APKs that are divided into different packages. Additionally, Google Play Protect may be blocking the installation if it detects any malicious code or nature. If you've added any third-party software to your device, such as custom ROMs, this could be causing the issue. You can also try resetting your device to factory settings if the problem persists.
Finally, if you're trying to install an APK from an external source, make sure it hasn't been contaminated with a virus. If you're trying to install the ExpressVPN app for Android, you'll need to enable APK installations on your device and download the APK file from the ExpressVPN website.