When installing an application APK manually on android phone, tablet or TV Box, the privacy protection of android OS will stop the process. Here is how you can allow permission for such operation.
The intervention of the android operating system to stop the execution of a 3rd-party, unknown APK file is due to the fact that it's an installation package outside Playstore environment and since it's not been installed or verified by Play Protect, it does not have the approval to pass android OS security checks. However, that's not the end of the world and you can easily permission such process. Remember, this is fine if you know the source of that APK file being genuine and safe. So act responsibly.
An example would clarify this better.
1-Say we want to download and manually install IPTV Smarters Pro APK. File downloaded so now let's open it.
2-Android Privacy Protection is stopping the process, identifying the APK file as unknown because it was not approved by Play Protect. However, you can install proceed with the process if you accept the risks and allow the permission. The risk would only be if you do not know the authenticity of the. In this case, file has been downloaded from WHMCS Smarters official website who are the owner of the application so there should be no risks.
3-Clicking on settings will bring us to Install Unknown Apps section under Privacy Protection settings. Click that switch so the permission is allwoed.
4-Check the consent box and click OK.
5-Now click Back button on the top left.
6-We are brought back where we left off. Now we can see that the Install has appeared as an option.
7-Installation has been successful and we can proceed with configuration of our app.
If you want, here you can see the whole process of side-loading IPTV Smarters pro and IPTV installation.