When I transfer .apk file in the download directory of my android 5.1 and click on it accepting all warning messages it install correctly the first time only.
If I uninstall and try to reinstall , then no installation result without any fault indicated.
Gogeling my problem give me this clue:
https://support.mobile-tracker-free.com/hc/fr/articles/360005346953-Comment-d%C3%A9sactiver-Google-Play-Protect-So .. If in Setting > Application > Google play store .... I disable ( It clear all cache associated)
Click on my apk. accepting all warning messages it install correctly.
Re enable Google play store