You need to turn off Unknown sources
Your Android device is able to install apps from multiple locations. For most users, the Google Play Store has plenty of apps. Some apps, such as apps developed by your company for internal use, may be installed from other marketplaces. Installing apps from unverified locations can lead to issues with your device, and can potentially cause security problems. Your company support has decided that, to make sure your corporate data is secure, to turn off access to "Unknown sources".
If you see the message, “Installation from Unknown sources must be disabled,” you need to go to Settings > Security (on some devices, this may be Lock screen and security instead), and turn off Unknown sources.
After you turn off Unknown sources, it may take a few minutes before you can continue to use your Android device to access company or school data.
Still need help? Contact your company support. For contact information, check the Company Portal website.