COMEAndroid's installation-source switch is unavailable
An unavailable installation-source setting can reflect device or profile restrictions. Read the reason before treating it as a COME file problem.
Check the selected source
Ensure you are looking at the app that actually opened the APK. A browser and file manager can have separate source permissions. A disabled control for another app may be unrelated to the current installation attempt.
Read device context
Look for administrator or profile messages and check the phone's normal help. Menu names vary. Do not assume that a hidden or greyed-out switch is a bug that can safely be bypassed.
Avoid workaround tools
Do not install unknown helpers, remove device management or switch off Play Protect to force access to a control. If the setting is restricted, follow the owner or administrator's route. Preserve any existing working installation.
A useful next step
Resolve the device-setting question before trying the APK again.
Related: Review source permission after a COME APK installation