"Can't open file" message when opening .apk from Downloads

[Android Central Question]

I created an application and sent the .apk file to my client. When he tries to open it he get "Can't open file".

He's device : Vodafone Smart X9, model VFD 820, running Android 8.1.0

I told him to enable "Unknown Sources" in Security Settings.

Can someone please give me advice on how he can open the .apk I sent?

On my Samsung J7 Pro, I just enable "Unknown Sources" then I can install the apk.

 

[B. Diddy]

I moved this to the Software Development forum for more specific traffic.