Hello! I understand that you're experiencing difficulties installing APK files that you downloaded from the file manager or other sources on your Vivo V15 device. However, you mentioned that apps from the Play Store can be installed without any issues. The error message you encounter says, "Can't install package while in secure FRP."
This error message usually occurs when the device is protected by Factory Reset Protection (FRP), which is a security feature designed to prevent unauthorized access to your phone after a factory reset. It seems that FRP is preventing you from installing APK files that are not from the Play Store.
To resolve this issue, you can try the following steps:
1. Disable FRP: Go to your device's settings, then navigate to "Security" or "Lock screen and security" options. Look for "Google Account Manager" or "Google Play Services" and disable it. This should disable FRP temporarily, allowing you to install APK files.
2. Enable Unknown Sources: In your device's settings, go to "Security" or "Lock screen and security" options. Look for "Unknown Sources" and enable it. This will allow you to install APK files from sources other than the Play Store.
3. Clear Cache and Data: Open the settings of your device, go to "Apps" or "Applications," then find the file manager you are using. Tap on it and select "Clear Cache" and "Clear Data." This will clear any temporary files or data related to the file manager, which may be causing the issue.
4. Try a Different File Manager: If the issue persists, try using a different file manager application to install the APK files. There are many file manager apps available on the Play Store that you can try.
If none of these solutions work, it may be worth contacting Vivo customer support or visiting a service center to get further assistance with the issue. They should be able to help you resolve the problem.