Apps that do not originate from the official Google Play Store cannot be installed on the Oppo smartphone without access rights, so the app must be explicitly allowed (for example, the file manager) to install an .apk app from an unknown source.

Android 10 | ColorOS 7.0
Step 1:  Open the  Settings
Step 2:  Tap on  Security
Step 3:  Tap on  Unknown source installations
Step 4:  Enable or Disable desired  App
Step 5:  Confirm with  Allow
  1. Open the Settings
  2. Tap on Security
  3. Tap on Unknown source installations
  4. Enable or Disable desired App
  5. Confirm with Allow

Oppo Instructions
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